feat: prevent adding duplicate library items

Block saving a new item when one already exists, so no duplicate is
created and existing progress is never overwritten. Matches by provider
id (external_source/external_id) when available, otherwise by media
type, year and title.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/Database.cpp

@@ -416,6 +416,32 @@ void Database::loadCustomFields(MediaItem &item) {
         item.customFields.insert(q.value(0).toString(), q.value(1).toString());
 }
 
+std::optional<Database::DuplicateMatch> Database::findDuplicate(const MediaItem &item) {
+    QSqlDatabase db = QSqlDatabase::database(m_connName);
+    QSqlQuery q(db);
+    // Provider-backed entries carry a stable external id; match on that first.
+    if (!item.externalId.isEmpty() && !item.externalSource.isEmpty()) {
+        q.prepare(QStringLiteral(
+            "SELECT id,title FROM media WHERE external_source=? AND external_id=? "
+            "AND id<>? LIMIT 1"));
+        q.addBindValue(item.externalSource);
+        q.addBindValue(item.externalId);
+        q.addBindValue(item.id);
+    } else {
+        // Manual entries: same media type, year and title (case-insensitive).
+        q.prepare(QStringLiteral(
+            "SELECT id,title FROM media WHERE type=? AND year=? "
+            "AND LOWER(TRIM(title))=LOWER(?) AND id<>? LIMIT 1"));
+        q.addBindValue(mediaTypeToString(item.type));
+        q.addBindValue(item.year);
+        q.addBindValue(item.title.trimmed());
+        q.addBindValue(item.id);
+    }
+    if (!q.exec() || !q.next())
+        return std::nullopt;
+    return DuplicateMatch{ q.value(0).toInt(), q.value(1).toString() };
+}
+
 std::optional<MediaItem> Database::loadItem(int id) {
     QSqlDatabase db = QSqlDatabase::database(m_connName);
     QSqlQuery q(db);

src/core/Database.h

@@ -43,6 +43,13 @@ public:
     bool deleteItem(int id);
     std::optional<MediaItem> loadItem(int id);
 
+    // Identifies an existing library item that a given item would duplicate.
+    struct DuplicateMatch { int id; QString title; };
+    // Returns such a match, or nullopt. The item's own id is excluded (so an
+    // item never matches itself). Matches by (external_source, external_id)
+    // when both are set, otherwise by (type, title, year).
+    std::optional<DuplicateMatch> findDuplicate(const MediaItem &item);
+
     // Returns items (with full segment trees) matching the criteria.
     QVector<MediaItem> queryItems(const FilterCriteria &c);
 

src/ui/EditDialog.cpp

@@ -455,6 +455,20 @@ void EditDialog::accept()
     }
     m_item.segments = rebuilt;
 
+    // Block adding an item that already exists, so no duplicate is created and
+    // no existing progress gets overwritten. Only new items are checked; an
+    // edit keeps its own id and is therefore never flagged.
+    if (m_item.id < 0) {
+        if (auto dup = m_db->findDuplicate(m_item)) {
+            QMessageBox::warning(this, windowTitle(),
+                QStringLiteral("„%1\" ist bereits in deiner Bibliothek vorhanden.\n\n"
+                               "Es wurde nichts hinzugefügt, damit dein bestehender "
+                               "Fortschritt nicht überschrieben wird.")
+                    .arg(dup->title));
+            return;
+        }
+    }
+
     if (!m_item.dateAdded.isValid())
         m_item.dateAdded = QDateTime::currentDateTime();