diff --git a/leggen/services/database_service.py b/leggen/services/database_service.py
index 5619930..b441e93 100644
--- a/leggen/services/database_service.py
+++ b/leggen/services/database_service.py
@@ -693,7 +693,7 @@ class DatabaseService:
 
             # Add the display_name column
             cursor.execute("""
-                ALTER TABLE accounts 
+                ALTER TABLE accounts
                 ADD COLUMN display_name TEXT
             """)
 
@@ -857,6 +857,14 @@ class DatabaseService:
 
             for transaction in transactions:
                 try:
+                    # Check if transaction already exists before insertion
+                    cursor.execute(
+                        """SELECT COUNT(*) FROM transactions
+                           WHERE accountId = ? AND transactionId = ?""",
+                        (transaction["accountId"], transaction["transactionId"]),
+                    )
+                    exists = cursor.fetchone()[0] > 0
+
                     cursor.execute(
                         insert_sql,
                         (
@@ -873,7 +881,11 @@ class DatabaseService:
                             json.dumps(transaction["rawTransaction"]),
                         ),
                     )
-                    new_transactions.append(transaction)
+
+                    # Only add to new_transactions if it didn't exist before
+                    if not exists:
+                        new_transactions.append(transaction)
+
                 except sqlite3.IntegrityError as e:
                     logger.warning(
                         f"Failed to insert transaction {transaction.get('transactionId')}: {e}"
diff --git a/tests/unit/test_database_service.py b/tests/unit/test_database_service.py
index 94f90f7..221d36f 100644
--- a/tests/unit/test_database_service.py
+++ b/tests/unit/test_database_service.py
@@ -324,6 +324,8 @@ class TestDatabaseService:
         with patch("sqlite3.connect") as mock_connect:
             mock_conn = mock_connect.return_value
             mock_cursor = mock_conn.cursor.return_value
+            # Mock fetchone to return (0,) indicating transaction doesn't exist yet
+            mock_cursor.fetchone.return_value = (0,)
 
             result = await database_service._persist_transactions_sqlite(
                 "test-account-123", sample_transactions_db_format
@@ -338,6 +340,29 @@ class TestDatabaseService:
             mock_conn.commit.assert_called_once()
             mock_conn.close.assert_called_once()
 
+    async def test_persist_transactions_sqlite_duplicate_detection(
+        self, database_service, sample_transactions_db_format
+    ):
+        """Test that existing transactions are not returned as new."""
+        with patch("sqlite3.connect") as mock_connect:
+            mock_conn = mock_connect.return_value
+            mock_cursor = mock_conn.cursor.return_value
+            # Mock fetchone to return (1,) indicating transaction already exists
+            mock_cursor.fetchone.return_value = (1,)
+
+            result = await database_service._persist_transactions_sqlite(
+                "test-account-123", sample_transactions_db_format
+            )
+
+            # Should return empty list since all transactions already exist
+            assert len(result) == 0
+
+            # Verify database operations still happened (INSERT OR REPLACE executed)
+            mock_connect.assert_called()
+            mock_cursor.execute.assert_called()
+            mock_conn.commit.assert_called_once()
+            mock_conn.close.assert_called_once()
+
     async def test_persist_transactions_sqlite_error(self, database_service):
         """Test handling error during transaction persistence."""
         with patch("sqlite3.connect") as mock_connect: