diff --git a/finance/app/routers/gui.py b/finance/app/routers/gui.py
index 0f2fe6e..36753f2 100644
--- a/finance/app/routers/gui.py
+++ b/finance/app/routers/gui.py
@@ -27,6 +27,7 @@ from app.routers.planning import recurring_suggestions as _recurring_suggestions
 from app.routers.scenarios import list_scenarios as _list_scenarios
 from app.routers.transactions import count_transactions, list_transactions, normalize_sort
 from app.services.balances import account_balance, total_balance
+from app.services.double_count import all_conflicts
 from app.services.planning_chart import refresh_planning_chart
 from app.services.projection_service import refresh_stale_projections
 from app.version import get_version
@@ -337,6 +338,11 @@ def planung_page(request: Request, session: Session = Depends(get_session)):
     recurring = _list_recurring(session=session)
     planned = _list_planned(session=session)
     loans = _list_loans(session=session)
+    doppelt_posten: dict[int, list[str]] = {}
+    doppelt_kredit: dict[int, list[str]] = {}
+    for k in all_conflicts(session):
+        doppelt_posten.setdefault(k.posten_id, []).append(k.kredit_name)
+        doppelt_kredit.setdefault(k.kredit_id, []).append(k.posten_name)
     return templates.TemplateResponse(request, "planning.html", {
         "categories": categories,
         "category_names": {c.id: c.name for c in categories},
@@ -347,6 +353,8 @@ def planung_page(request: Request, session: Session = Depends(get_session)):
         "loans": loans,
         "rhythms": ["monthly", "quarterly", "yearly"],
         "repayment_types": ["annuity", "bullet"],
+        "doppelt_posten": doppelt_posten,
+        "doppelt_kredit": doppelt_kredit,
     })
 
 
diff --git a/finance/app/static/doppelzaehlung.js b/finance/app/static/doppelzaehlung.js
new file mode 100644
index 0000000..cb63243
--- /dev/null
+++ b/finance/app/static/doppelzaehlung.js
@@ -0,0 +1,89 @@
+/* Warnfenster bei möglicher Doppelzählung Kredit ↔ Fixposten (Ausbaustufe 12).
+ *
+ * Die API antwortet mit 409 und detail.code "doppelzaehlung_kredit", wenn ein
+ * Posten vermutlich einer Kreditrate entspricht. handle() fängt genau diesen
+ * Fall im globalen htmx:responseError-Handler ab (alle anderen Fehler laufen
+ * weiter über den alert) und zeigt den <dialog>. „Trotzdem speichern" markiert
+ * das auslösende Formular (data-doppelzaehlung-bestaetigt="1") und sendet es
+ * erneut; die json-form-Extension ergänzt dann doppelzaehlung_bestaetigt=true.
+ * Nach jedem Request wird die Markierung entfernt, damit ein späteres
+ * Speichern die Warnung nicht stillschweigend umgeht.
+ */
+(function () {
+  'use strict';
+
+  var CODE = 'doppelzaehlung_kredit';
+  var ATTR = 'data-doppelzaehlung-bestaetigt';
+  var fmt = new Intl.NumberFormat('de-DE', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
+
+  function parseDetail(xhr) {
+    if (!xhr || xhr.status !== 409) { return null; }
+    var body;
+    try { body = JSON.parse(xhr.responseText); } catch (e) { return null; }
+    var d = body && body.detail;
+    return d && d.code === CODE ? d : null;
+  }
+
+  function eur(s) { return fmt.format(parseFloat(s)) + ' €'; }
+
+  function konfliktText(k) {
+    return 'Posten „' + k.posten + '“ (' + eur(k.posten_betrag) + ') ↔ Kredit „'
+      + k.kredit + '“ (Rate ' + eur(k.kredit_rate) + ')';
+  }
+
+  function handle(evt) {
+    var d = parseDetail(evt && evt.detail && evt.detail.xhr);
+    var elt = evt && evt.detail && evt.detail.elt;
+    var form = elt && elt.closest ? elt.closest('form') : null;
+    if (!d || !form) { return false; }
+    var doc = form.ownerDocument;
+    var dlg = doc.getElementById('doppelzaehlung-dialog');
+    if (!dlg) { return false; }
+    dlg.querySelector('.dz-meldung').textContent = d.meldung;
+    var list = dlg.querySelector('.dz-konflikte');
+    list.textContent = '';
+    (d.konflikte || []).forEach(function (k) {
+      var li = doc.createElement('li');
+      li.textContent = konfliktText(k);
+      list.appendChild(li);
+    });
+    dlg.fbForm = form;
+    dlg.showModal();
+    return true;
+  }
+
+  function abbrechen(dlg) {
+    dlg.fbForm = null;
+    dlg.close();
+  }
+
+  function bestaetigen(dlg) {
+    var form = dlg.fbForm;
+    dlg.fbForm = null;
+    dlg.close();
+    if (!form) { return; }
+    form.setAttribute(ATTR, '1');
+    form.ownerDocument.defaultView.htmx.trigger(form, 'submit');
+  }
+
+  function init(doc) {
+    var dlg = doc.getElementById('doppelzaehlung-dialog');
+    if (!dlg) { return; }
+    doc.getElementById('dz-abbrechen').addEventListener('click', function () { abbrechen(dlg); });
+    doc.getElementById('dz-bestaetigen').addEventListener('click', function () { bestaetigen(dlg); });
+    doc.body.addEventListener('htmx:afterRequest', function (e) {
+      var elt = e.detail && e.detail.elt;
+      var form = elt && elt.closest ? elt.closest('form') : null;
+      if (form && form.getAttribute(ATTR) === '1') {
+        // Erst nach dem Request mit Flag entfernen: der 409-Request selbst
+        // hat die Markierung nie gesetzt.
+        form.removeAttribute(ATTR);
+      }
+    });
+  }
+
+  var api = { parseDetail: parseDetail, konfliktText: konfliktText, handle: handle,
+              abbrechen: abbrechen, bestaetigen: bestaetigen, init: init };
+  if (typeof module !== 'undefined' && module.exports) { module.exports = api; }
+  if (typeof window !== 'undefined') { window.FBDoppel = api; }
+})();
diff --git a/finance/app/static/style.css b/finance/app/static/style.css
index a999fc4..55b7e93 100644
--- a/finance/app/static/style.css
+++ b/finance/app/static/style.css
@@ -226,3 +226,13 @@ label:has(input:disabled), label:has(select:disabled) {
   color: inherit;
   font-weight: bold;
 }
+
+/* Doppelzählung Kredit/Fixposten (Ausbaustufe 12). */
+.warn-hint {
+  display: block;
+  font-size: 0.8em;
+  color: #b45309;
+}
+#doppelzaehlung-dialog {
+  max-width: 32rem;
+}
diff --git a/finance/app/templates/base.html b/finance/app/templates/base.html
index d91bb4b..663e4ff 100644
--- a/finance/app/templates/base.html
+++ b/finance/app/templates/base.html
@@ -7,6 +7,7 @@
   <link rel="stylesheet" href="/static/style.css">
   <script src="/static/htmx.min.js"></script>
   <script src="/static/sort.js"></script>
+  <script src="/static/doppelzaehlung.js"></script>
 </head>
 <body>
   <nav>
@@ -26,8 +27,18 @@
   <main>
     {% block content %}{% endblock %}
   </main>
+  <dialog id="doppelzaehlung-dialog">
+    <h2>Mögliche Doppelzählung</h2>
+    <p class="dz-meldung"></p>
+    <ul class="dz-konflikte"></ul>
+    <p>Trotzdem speichern?</p>
+    <button type="button" id="dz-abbrechen" autofocus>Abbrechen</button>
+    <button type="button" id="dz-bestaetigen">Trotzdem speichern</button>
+  </dialog>
   <script>
     document.body.addEventListener('htmx:responseError', function (e) {
+      // Mögliche Doppelzählung Kredit/Fixposten: eigenes Warnfenster statt alert.
+      if (window.FBDoppel && window.FBDoppel.handle(e)) { return; }
       alert('Fehler: ' + (e.detail.xhr.responseText || e.detail.xhr.status));
     });
 
@@ -72,6 +83,10 @@
             out[field.name] = field.checked;
           });
         }
+        // Bestätigte Doppelzählung (Warnfenster „Trotzdem speichern"):
+        if (elt.getAttribute && elt.getAttribute('data-doppelzaehlung-bestaetigt') === '1') {
+          out.doppelzaehlung_bestaetigt = true;
+        }
         xhr.overrideMimeType('text/json');
         return JSON.stringify(out);
       }
@@ -82,6 +97,8 @@
       document.getElementById(prefix + '-row-' + id).hidden = editing;
       document.getElementById(prefix + '-edit-' + id).hidden = !editing;
     }
+
+    if (window.FBDoppel) { window.FBDoppel.init(document); }
   </script>
   <footer class="version">Finanzberatungs-Tool v{{ app_version }}</footer>
 </body>
diff --git a/finance/app/templates/planning.html b/finance/app/templates/planning.html
index 71e3003..b25bd50 100644
--- a/finance/app/templates/planning.html
+++ b/finance/app/templates/planning.html
@@ -12,7 +12,7 @@
     <tbody>
       {% for r in recurring %}
       <tr id="rec-row-{{ r.id }}">
-        <td>{{ r.name }}</td>
+        <td data-sort="{{ r.name }}">{{ r.name }}{% for kn in doppelt_posten.get(r.id, []) %}<span class="warn-hint">⚠ zählt evtl. doppelt mit Kredit „{{ kn }}“</span>{% endfor %}</td>
         <td class="{{ 'neg' if r.amount < 0 else '' }}" data-sort="{{ r.amount }}">{{ r.amount|eur }} €</td>
         <td>{{ r.rhythm|de_label }}</td>
         <td>{{ r.due_day }}</td>
@@ -197,7 +197,7 @@
     <tbody>
       {% for l in loans %}
       <tr id="loan-row-{{ l.id }}">
-        <td>{{ l.name }}</td>
+        <td data-sort="{{ l.name }}">{{ l.name }}{% for pn in doppelt_kredit.get(l.id, []) %}<span class="warn-hint">⚠ zählt evtl. doppelt mit Posten „{{ pn }}“</span>{% endfor %}</td>
         <td data-sort="{{ l.principal }}">{{ l.principal|eur }} €</td>
         <td>{{ l.annual_rate_pct|eur }} %</td>
         <td data-sort="{{ l.term_months }}">{{ l.term_months }} Monate</td>
diff --git a/finance/tests/js/doppelzaehlung.dom.test.js b/finance/tests/js/doppelzaehlung.dom.test.js
new file mode 100644
index 0000000..3046713
--- /dev/null
+++ b/finance/tests/js/doppelzaehlung.dom.test.js
@@ -0,0 +1,75 @@
+'use strict';
+const test = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+
+let JSDOM = null;
+try { ({ JSDOM } = require('jsdom')); } catch (e) { /* jsdom fehlt -> skip */ }
+const SRC = fs.readFileSync(path.join(__dirname, '../../app/static/doppelzaehlung.js'), 'utf8');
+
+const HTML = `
+<form id="f" hx-post="/api/recurring"><input name="name" value="Rate Auto"></form>
+<dialog id="doppelzaehlung-dialog">
+  <p class="dz-meldung"></p><ul class="dz-konflikte"></ul>
+  <button type="button" id="dz-abbrechen">Abbrechen</button>
+  <button type="button" id="dz-bestaetigen">Trotzdem speichern</button>
+</dialog>`;
+const BODY = JSON.stringify({ detail: { code: 'doppelzaehlung_kredit', meldung: 'Erklärung',
+  konflikte: [{ posten: 'Rate Auto', posten_betrag: '-100.00', kredit: 'Auto', kredit_rate: '100.00' }] } });
+
+function load() {
+  const dom = new JSDOM(`<!doctype html><body>${HTML}</body>`, { runScripts: 'outside-only' });
+  const w = dom.window;
+  const calls = [];
+  w.htmx = { trigger: (el, name) => calls.push([el.id, name]) };
+  const dlg = w.document.getElementById('doppelzaehlung-dialog');
+  dlg.showModal = function () { this.setAttribute('open', ''); };
+  dlg.close = function () { this.removeAttribute('open'); };
+  w.eval(SRC);
+  w.FBDoppel.init(w.document);
+  const form = w.document.getElementById('f');
+  const evt = { detail: { xhr: { status: 409, responseText: BODY }, elt: form } };
+  return { w, dlg, form, evt, calls };
+}
+
+test('409 öffnet Dialog mit Erklärung und Konfliktliste', { skip: !JSDOM }, () => {
+  const { w, dlg, evt } = load();
+  assert.equal(w.FBDoppel.handle(evt), true);
+  assert.ok(dlg.hasAttribute('open'));
+  assert.equal(dlg.querySelector('.dz-meldung').textContent, 'Erklärung');
+  assert.equal(dlg.querySelectorAll('.dz-konflikte li').length, 1);
+});
+
+test('andere Fehler werden nicht abgefangen', { skip: !JSDOM }, () => {
+  const { w, form } = load();
+  assert.equal(w.FBDoppel.handle({ detail: { xhr: { status: 422, responseText: '{}' }, elt: form } }), false);
+});
+
+test('Abbrechen speichert nicht', { skip: !JSDOM }, () => {
+  const { w, dlg, form, evt, calls } = load();
+  w.FBDoppel.handle(evt);
+  w.document.getElementById('dz-abbrechen').click();
+  assert.ok(!dlg.hasAttribute('open'));
+  assert.equal(calls.length, 0);
+  assert.ok(!form.hasAttribute('data-doppelzaehlung-bestaetigt'));
+  assert.equal(form.querySelector('input').value, 'Rate Auto');
+});
+
+test('Trotzdem speichern sendet erneut mit Flag', { skip: !JSDOM }, () => {
+  const { w, dlg, form, evt, calls } = load();
+  w.FBDoppel.handle(evt);
+  w.document.getElementById('dz-bestaetigen').click();
+  assert.ok(!dlg.hasAttribute('open'));
+  assert.equal(form.getAttribute('data-doppelzaehlung-bestaetigt'), '1');
+  assert.deepEqual(calls, [['f', 'submit']]);
+});
+
+test('Flag nach Request entfernt', { skip: !JSDOM }, () => {
+  const { w, form, evt } = load();
+  w.FBDoppel.handle(evt);
+  w.document.getElementById('dz-bestaetigen').click();
+  const e = new w.CustomEvent('htmx:afterRequest', { bubbles: true, detail: { elt: form } });
+  form.dispatchEvent(e);
+  assert.ok(!form.hasAttribute('data-doppelzaehlung-bestaetigt'));
+});
diff --git a/finance/tests/js/doppelzaehlung.test.js b/finance/tests/js/doppelzaehlung.test.js
new file mode 100644
index 0000000..3aa66bd
--- /dev/null
+++ b/finance/tests/js/doppelzaehlung.test.js
@@ -0,0 +1,20 @@
+'use strict';
+const test = require('node:test');
+const assert = require('node:assert');
+const D = require('../../app/static/doppelzaehlung.js');
+
+const body = { detail: { code: 'doppelzaehlung_kredit', meldung: 'M',
+  konflikte: [{ posten: 'Rate Auto', posten_betrag: '-1234.50', kredit: 'Auto', kredit_rate: '1234.50' }] } };
+
+test('parseDetail erkennt nur 409 mit Code', () => {
+  assert.equal(D.parseDetail({ status: 409, responseText: JSON.stringify(body) }).code, 'doppelzaehlung_kredit');
+  assert.equal(D.parseDetail({ status: 409, responseText: '{"detail":"Kategorie existiert bereits"}' }), null);
+  assert.equal(D.parseDetail({ status: 422, responseText: JSON.stringify(body) }), null);
+  assert.equal(D.parseDetail({ status: 409, responseText: 'kein json' }), null);
+  assert.equal(D.parseDetail(null), null);
+});
+
+test('konfliktText deutsch formatiert', () => {
+  assert.equal(D.konfliktText(body.detail.konflikte[0]),
+    'Posten „Rate Auto“ (-1.234,50 €) ↔ Kredit „Auto“ (Rate 1.234,50 €)');
+});
diff --git a/finance/tests/test_gui.py b/finance/tests/test_gui.py
index 9932bb4..84e3948 100644
--- a/finance/tests/test_gui.py
+++ b/finance/tests/test_gui.py
@@ -588,3 +588,46 @@ def test_planungs_refresh_fehler_bricht_seiten_nicht(client, monkeypatch):
     client.post("/login", data={"username": "admin", "password": "geheim"})
     for path in ("/", "/planung"):
         assert client.get(path).status_code == 200, path
+
+
+def test_base_hat_doppelzaehlungs_dialog(client):
+    client.post("/login", data={"username": "admin", "password": "geheim"})
+    html = client.get("/").text
+    assert '<script src="/static/doppelzaehlung.js"></script>' in html
+    assert '<dialog id="doppelzaehlung-dialog">' in html
+    assert 'id="dz-abbrechen" autofocus' in html
+    assert ">Trotzdem speichern</button>" in html
+    assert "FBDoppel.handle(e)" in html
+    assert "doppelzaehlung_bestaetigt = true" in html
+
+
+def test_planung_zeigt_doppelzaehlungs_hinweis(client, db):
+    client.post("/login", data={"username": "admin", "password": "geheim"})
+    db.add_all([
+        Loan(name="Auto", principal=Decimal("1200.00"), annual_rate_pct=Decimal("0"),
+             term_months=12, payout_date=date(2026, 1, 15), repayment_type="annuity"),
+        RecurringItem(name="Rate Auto", amount=Decimal("-100.00"), rhythm="monthly", due_day=15),
+        RecurringItem(name="Miete", amount=Decimal("-600.00"), rhythm="monthly", due_day=1),
+    ])
+    db.commit()
+    html = client.get("/planung").text
+    assert "⚠ zählt evtl. doppelt mit Kredit „Auto“" in html
+    assert "⚠ zählt evtl. doppelt mit Posten „Rate Auto“" in html
+    assert html.count("⚠ zählt evtl. doppelt") == 2
+    assert '<td data-sort="Rate Auto">' in html
+    assert '<td data-sort="Auto">' in html
+
+
+def test_planung_ohne_konflikt_kein_hinweis(client, db):
+    client.post("/login", data={"username": "admin", "password": "geheim"})
+    db.add(RecurringItem(name="Miete", amount=Decimal("-600.00"), rhythm="monthly", due_day=1))
+    db.commit()
+    assert "zählt evtl. doppelt" not in client.get("/planung").text
+
+
+def test_vorschlag_formular_nutzt_json_form_post(client):
+    # Review Focus 5: „Vorschlag übernehmen" läuft über denselben Warnweg.
+    import re
+    from pathlib import Path
+    src = Path("app/templates/planning.html").read_text(encoding="utf-8")
+    assert re.search(r'class="inline-form" hx-ext="json-form" hx-post="/api/recurring"', src)