backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -128,6 +128,8 @@ public enum ErrorCode {
     JOURNEY_ITEM_POSITION_CONFLICT,
     /** The journey already has the maximum allowed number of items (100). 400 */
     JOURNEY_AT_CAPACITY,
+    /** The document is already present in this journey — duplicate items are not allowed. 409 */
+    JOURNEY_DOCUMENT_ALREADY_ADDED,
     /** The Geschichte is not of type JOURNEY — journey-item operations are not allowed on it. 400 */
     GESCHICHTE_TYPE_MISMATCH,
 

backend/src/main/java/org/raddatz/familienarchiv/geschichte/journeyitem/JourneyItemRepository.java

@@ -30,6 +30,9 @@ public interface JourneyItemRepository extends JpaRepository<JourneyItem, UUID>
     /** COUNT for the 100-item cap check — COUNT(*)-based, never MAX(position)-derived. */
     long countByGeschichteId(UUID geschichteId);
 
+    /** Dedup guard: true when the document is already linked to this journey. */
+    boolean existsByGeschichteIdAndDocumentId(UUID geschichteId, UUID documentId);
+
     /**
      * Loads journey items with their linked Document in a single JOIN FETCH query,
      * eliminating the N+1 SELECT that would occur when accessing item.getDocument()

backend/src/main/java/org/raddatz/familienarchiv/geschichte/journeyitem/JourneyItemService.java

@@ -69,6 +69,10 @@ public class JourneyItemService {
 
         Document doc = null;
         if (dto.getDocumentId() != null) {
+            if (journeyItemRepository.existsByGeschichteIdAndDocumentId(geschichteId, dto.getDocumentId())) {
+                throw DomainException.conflict(ErrorCode.JOURNEY_DOCUMENT_ALREADY_ADDED,
+                        "Document already in journey: " + dto.getDocumentId());
+            }
             doc = documentService.findSummaryByIdInternal(dto.getDocumentId());
         }
 

backend/src/test/java/org/raddatz/familienarchiv/geschichte/journeyitem/JourneyItemServiceTest.java

@@ -288,6 +288,22 @@ class JourneyItemServiceTest {
                         .isEqualTo(ErrorCode.JOURNEY_AT_CAPACITY));
     }
 
+    @Test
+    void append_returns409_when_document_already_in_journey() {
+        Geschichte journey = journey(geschichteId);
+        when(geschichteQueryService.findById(geschichteId)).thenReturn(Optional.of(journey));
+        when(journeyItemRepository.countByGeschichteId(geschichteId)).thenReturn(1L);
+        when(journeyItemRepository.existsByGeschichteIdAndDocumentId(geschichteId, docId)).thenReturn(true);
+
+        JourneyItemCreateDTO dto = new JourneyItemCreateDTO();
+        dto.setDocumentId(docId);
+
+        assertThatThrownBy(() -> journeyItemService.append(geschichteId, dto))
+                .isInstanceOf(DomainException.class)
+                .satisfies(e -> assertThat(((DomainException) e).getCode())
+                        .isEqualTo(ErrorCode.JOURNEY_DOCUMENT_ALREADY_ADDED));
+    }
+
     @Test
     void cap_is_COUNT_based_not_MAX_position_based() {
         // 99 rows with MAX(position)=2000 should still accept the 100th append