diff --git a/apps/api/src/routes/cards.ts b/apps/api/src/routes/cards.ts
index 77d1e7f..5fe9a9c 100644
--- a/apps/api/src/routes/cards.ts
+++ b/apps/api/src/routes/cards.ts
@@ -4,6 +4,7 @@ import { Hono } from 'hono';
 import {
 	CardCreateSchema,
 	CardUpdateSchema,
+	cardContentHash,
 	newReview,
 	subIndexCount,
 	subIndexCountForCloze,
@@ -66,6 +67,10 @@ export function cardsRouter(deps: CardsDeps = {}): Hono<{ Variables: AuthVars }>
 		const cardId = ulid();
 		const now = new Date();
 		const subIndices = Array.from({ length: count }, (_, i) => i);
+		const contentHash = await cardContentHash({
+			type: parsed.data.type,
+			fields: parsed.data.fields,
+		});
 
 		const [cardRow] = await dbOf().transaction(async (tx) => {
 			const [card] = await tx
@@ -77,6 +82,7 @@ export function cardsRouter(deps: CardsDeps = {}): Hono<{ Variables: AuthVars }>
 					type: parsed.data.type,
 					fields: parsed.data.fields,
 					mediaRefs: parsed.data.media_refs ?? [],
+					contentHash,
 					createdAt: now,
 					updatedAt: now,
 				})
@@ -120,6 +126,24 @@ export function cardsRouter(deps: CardsDeps = {}): Hono<{ Variables: AuthVars }>
 		return c.json({ cards: rows.map(toCardDto), total: rows.length });
 	});
 
+	/**
+	 * Liefert nur die content_hash-Liste des Users — kompakter Pfad für
+	 * den Anki-Re-Import-Dedupe. Frontend lädt das einmal und prüft pro
+	 * Karte clientseitig, statt für jeden Insert einen Round-Trip zu
+	 * machen. Karten ohne content_hash (Pre-Phase-9j) werden weggefiltert.
+	 */
+	r.get('/hashes', async (c) => {
+		const userId = c.get('userId');
+		const rows = await dbOf()
+			.select({ contentHash: cards.contentHash })
+			.from(cards)
+			.where(eq(cards.userId, userId));
+		const hashes = rows
+			.map((r) => r.contentHash)
+			.filter((h): h is string => typeof h === 'string' && h.length > 0);
+		return c.json({ hashes, total: hashes.length });
+	});
+
 	r.get('/:id', async (c) => {
 		const userId = c.get('userId');
 		const id = c.req.param('id');
diff --git a/apps/api/tests/cards.test.ts b/apps/api/tests/cards.test.ts
index 0658d22..df637ef 100644
--- a/apps/api/tests/cards.test.ts
+++ b/apps/api/tests/cards.test.ts
@@ -28,6 +28,12 @@ describe('cardsRouter — auth-gate', () => {
 		const res = await app.request('/api/v1/cards');
 		expect(res.status).toBe(401);
 	});
+
+	it('GET /hashes ohne X-User-Id ist 401', async () => {
+		const { app } = buildApp();
+		const res = await app.request('/api/v1/cards/hashes');
+		expect(res.status).toBe(401);
+	});
 });
 
 describe('cardsRouter — Input-Validation', () => {
diff --git a/apps/web/src/lib/anki/import.ts b/apps/web/src/lib/anki/import.ts
index 45442b1..def34ec 100644
--- a/apps/web/src/lib/anki/import.ts
+++ b/apps/web/src/lib/anki/import.ts
@@ -9,16 +9,24 @@
  * Phase-8-MVP: Bilder + Audio werden gedroppt (siehe parse.ts
  * `sanitizeAnkiHtml`). Ein späterer Media-Pfad ist additiv.
  *
- * No de-dupe: Re-Import derselben .apkg legt doppelte Decks an.
+ * Phase-9j-Re-Import-Dedupe: Vor dem Insert wird der content_hash der
+ * Karte berechnet (gleiche Funktion wie der Server) und gegen die
+ * existierende Hash-Liste des Users geprüft. Duplikate werden gezählt
+ * und übersprungen — Re-Imports bringen also keine doppelten Karten
+ * mehr ins Deck. Decks werden nicht dedupliziert (gewollt: zwei
+ * .apkg-Files mit identischen Decknamen sollen sich nicht
+ * versehentlich zusammenführen).
  */
 
+import { cardContentHash } from '@cards/domain';
 import { createDeck } from '$lib/api/decks.ts';
-import { createCard } from '$lib/api/cards.ts';
+import { createCard, listCardHashes } from '$lib/api/cards.ts';
 import { sanitizeAnkiHtml, type ParsedAnki } from './parse.ts';
 
 export interface ImportResult {
 	decksCreated: number;
 	cardsCreated: number;
+	cardsSkippedDuplicate: number;
 	failed: number;
 	failures: string[];
 }
@@ -36,10 +44,22 @@ export async function importParsedAnki(
 	const result: ImportResult = {
 		decksCreated: 0,
 		cardsCreated: 0,
+		cardsSkippedDuplicate: 0,
 		failed: 0,
 		failures: [],
 	};
 
+	// Vor dem Insert die Hash-Liste des Users laden — wenn der Endpoint
+	// fehlschlägt (z.B. älterer Server vor Phase 9j), fallen wir
+	// stillschweigend auf "kein Dedupe" zurück.
+	const existingHashes = new Set<string>();
+	try {
+		const r = await listCardHashes();
+		for (const h of r.hashes) existingHashes.add(h);
+	} catch {
+		// Dedupe bleibt aus — Karten werden eingefügt wie zuvor.
+	}
+
 	// 1) Decks — Anki "::"-Hierarchie zu " / "-Strings flach machen.
 	const ankiIdToDeckId = new Map<string, string>();
 	let deckIdx = 0;
@@ -71,11 +91,22 @@ export async function importParsedAnki(
 		}
 	};
 
-	// 2) Cards — Felder sanitizen (Media-Refs werden gedroppt).
+	// 2) Cards — Felder sanitizen, content_hash prüfen, einfügen.
 	for (let i = 0; i < parsed.cards.length; i++) {
 		opts.onProgress?.({ stage: 'cards', current: i, total: parsed.cards.length });
 		const card = parsed.cards[i];
 
+		const cleanFields: Record<string, string> = {};
+		for (const [key, value] of Object.entries(card.fields)) {
+			cleanFields[key] = sanitizeAnkiHtml(value);
+		}
+
+		const hash = await cardContentHash({ type: card.type, fields: cleanFields });
+		if (existingHashes.has(hash)) {
+			result.cardsSkippedDuplicate++;
+			continue;
+		}
+
 		let targetDeckId = ankiIdToDeckId.get(card.ankiDeckId);
 		if (!targetDeckId) {
 			const fallback = await ensureFallbackDeck();
@@ -86,11 +117,6 @@ export async function importParsedAnki(
 			targetDeckId = fallback;
 		}
 
-		const cleanFields: Record<string, string> = {};
-		for (const [key, value] of Object.entries(card.fields)) {
-			cleanFields[key] = sanitizeAnkiHtml(value);
-		}
-
 		try {
 			await createCard({
 				deck_id: targetDeckId,
@@ -98,6 +124,9 @@ export async function importParsedAnki(
 				fields: cleanFields,
 			});
 			result.cardsCreated++;
+			// Hash sofort merken — derselbe Import könnte zwei identische
+			// Karten enthalten (Anki-Drift), zweite würde sonst auch rein.
+			existingHashes.add(hash);
 		} catch (e) {
 			result.failed++;
 			result.failures.push(`card "${preview(cleanFields)}": ${errMessage(e)}`);
diff --git a/apps/web/src/lib/api/cards.ts b/apps/web/src/lib/api/cards.ts
index 984ce0f..34ac16a 100644
--- a/apps/web/src/lib/api/cards.ts
+++ b/apps/web/src/lib/api/cards.ts
@@ -6,6 +6,11 @@ export function listCards(deckId?: string) {
 	return api<{ cards: Card[]; total: number }>(`/api/v1/cards${qs}`);
 }
 
+/** Holt nur die content_hash-Liste — kompakt für Anki-Re-Import-Dedupe. */
+export function listCardHashes() {
+	return api<{ hashes: string[]; total: number }>('/api/v1/cards/hashes');
+}
+
 export function getCard(id: string) {
 	return api<Card>(`/api/v1/cards/${id}`);
 }
diff --git a/apps/web/src/lib/components/AnkiImport.svelte b/apps/web/src/lib/components/AnkiImport.svelte
index 733d407..6ce514a 100644
--- a/apps/web/src/lib/components/AnkiImport.svelte
+++ b/apps/web/src/lib/components/AnkiImport.svelte
@@ -199,6 +199,11 @@
 					? t('import.done_summary_one', { cards: result.cardsCreated })
 					: t('import.done_summary', { cards: result.cardsCreated, decks: result.decksCreated })}
 			</div>
+			{#if result.cardsSkippedDuplicate > 0}
+				<div class="text-[var(--color-muted)]">
+					{t('import.done_dupes', { n: result.cardsSkippedDuplicate })}
+				</div>
+			{/if}
 			{#if result.failed > 0}
 				<details class="text-[var(--color-danger)]">
 					<summary class="cursor-pointer">{t('import.done_failures', { n: result.failed })}</summary>
diff --git a/apps/web/src/lib/i18n/de.ts b/apps/web/src/lib/i18n/de.ts
index 7f8589a..d4e2b32 100644
--- a/apps/web/src/lib/i18n/de.ts
+++ b/apps/web/src/lib/i18n/de.ts
@@ -156,6 +156,7 @@ export const de: TranslationNode = {
 		stage_done: 'Fertig.',
 		done_summary_one: '✓ {cards} Karten in 1 Deck angelegt.',
 		done_summary: '✓ {cards} Karten in {decks} Decks angelegt.',
+		done_dupes: '{n} Duplikate übersprungen (gleicher Inhalt schon vorhanden).',
 		done_failures: '{n} Fehler',
 		done_more: 'Weitere Datei',
 		error_label: 'Fehler: {msg}',
diff --git a/apps/web/src/lib/i18n/en.ts b/apps/web/src/lib/i18n/en.ts
index 2af2915..93f6f6f 100644
--- a/apps/web/src/lib/i18n/en.ts
+++ b/apps/web/src/lib/i18n/en.ts
@@ -153,6 +153,7 @@ export const en: TranslationNode = {
 		stage_done: 'Done.',
 		done_summary_one: '✓ {cards} cards in 1 deck.',
 		done_summary: '✓ {cards} cards in {decks} decks.',
+		done_dupes: '{n} duplicates skipped (same content already exists).',
 		done_failures: '{n} errors',
 		done_more: 'Another file',
 		error_label: 'Error: {msg}',
diff --git a/packages/cards-domain/src/content-hash.ts b/packages/cards-domain/src/content-hash.ts
new file mode 100644
index 0000000..acc3203
--- /dev/null
+++ b/packages/cards-domain/src/content-hash.ts
@@ -0,0 +1,35 @@
+/**
+ * Content-Hash für Karten — deterministisch, idempotent.
+ *
+ * Wird beim Card-Insert geschrieben (in `cards.content_hash`) und vom
+ * Anki-Re-Import zur Dedupe genutzt: zwei Imports derselben Karte
+ * landen nicht mehrfach im Deck.
+ *
+ * Eingabe: `type` + `fields`. Field-Keys werden sortiert, damit die
+ * Reihenfolge der Eingabe nichts ändert. Field-Values werden roh
+ * gehasht — Markdown-Whitespace und Cloze-Markup zählen mit
+ * (gewollt: zwei Karten mit demselben Text aber unterschiedlichem
+ * `{{c1::…}}`-Markup sind verschiedene Karten).
+ *
+ * Hash: SHA-256 → hex-string. Pure Web-Crypto (Browser + Bun + Node 20+).
+ */
+
+export interface CardContentInput {
+	type: string;
+	fields: Record<string, string>;
+}
+
+function canonicalize(input: CardContentInput): string {
+	const keys = Object.keys(input.fields).sort();
+	const fields = keys.map((k) => [k, input.fields[k] ?? '']);
+	return JSON.stringify({ type: input.type, fields });
+}
+
+export async function cardContentHash(input: CardContentInput): Promise<string> {
+	const text = canonicalize(input);
+	const data = new TextEncoder().encode(text);
+	const buf = await crypto.subtle.digest('SHA-256', data);
+	return Array.from(new Uint8Array(buf))
+		.map((b) => b.toString(16).padStart(2, '0'))
+		.join('');
+}
diff --git a/packages/cards-domain/src/index.ts b/packages/cards-domain/src/index.ts
index e6096fb..b2c8124 100644
--- a/packages/cards-domain/src/index.ts
+++ b/packages/cards-domain/src/index.ts
@@ -11,3 +11,4 @@ export * from './schemas/index.ts';
 export * from './fsrs.ts';
 export * from './protocol/index.ts';
 export * from './cloze.ts';
+export * from './content-hash.ts';
diff --git a/packages/cards-domain/tests/content-hash.test.ts b/packages/cards-domain/tests/content-hash.test.ts
new file mode 100644
index 0000000..61b1b61
--- /dev/null
+++ b/packages/cards-domain/tests/content-hash.test.ts
@@ -0,0 +1,61 @@
+import { describe, it, expect } from 'vitest';
+
+import { cardContentHash } from '../src/content-hash.ts';
+
+describe('cardContentHash', () => {
+	it('liefert deterministischen 64-char-hex-String (SHA-256)', async () => {
+		const h = await cardContentHash({
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+		});
+		expect(h).toMatch(/^[0-9a-f]{64}$/);
+	});
+
+	it('ist invariant gegenüber Field-Reihenfolge', async () => {
+		const a = await cardContentHash({
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+		});
+		const b = await cardContentHash({
+			type: 'basic',
+			fields: { back: 'A', front: 'Q' },
+		});
+		expect(a).toBe(b);
+	});
+
+	it('unterscheidet basic und basic-reverse', async () => {
+		const a = await cardContentHash({
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+		});
+		const b = await cardContentHash({
+			type: 'basic-reverse',
+			fields: { front: 'Q', back: 'A' },
+		});
+		expect(a).not.toBe(b);
+	});
+
+	it('unterscheidet zwei Cloze-Karten mit unterschiedlichem Cluster-Markup', async () => {
+		const a = await cardContentHash({
+			type: 'cloze',
+			fields: { text: 'Die {{c1::Hauptstadt}} ist {{c2::Paris}}.' },
+		});
+		const b = await cardContentHash({
+			type: 'cloze',
+			fields: { text: 'Die Hauptstadt ist {{c1::Paris}}.' },
+		});
+		expect(a).not.toBe(b);
+	});
+
+	it('unterscheidet Karten mit Whitespace-Drift', async () => {
+		const a = await cardContentHash({
+			type: 'basic',
+			fields: { front: 'Q', back: 'A' },
+		});
+		const b = await cardContentHash({
+			type: 'basic',
+			fields: { front: 'Q ', back: 'A' },
+		});
+		expect(a).not.toBe(b);
+	});
+});