[NOTICKET] add duplicate check for storing database

src/database_client/database_client_service.py

@@ -16,9 +16,46 @@ from src.utils.db_credential_encryption import (
 logger = get_logger("database_client_service")
 
 
+    # Fields that identify the same physical database per db_type.
+_CONNECTION_IDENTITY_KEYS: dict[str, tuple[str, ...]] = {
+    "postgres": ("host", "port", "database"),
+    "supabase": ("host", "port", "database"),
+    "mysql": ("host", "port", "database"),
+    "sqlserver": ("host", "port", "database"),
+    "bigquery": ("project_id", "dataset_id"),
+    "snowflake": ("account", "warehouse", "database"),
+}
+
+
 class DatabaseClientService:
     """Service for managing user-registered external database connections."""
 
+    async def _find_duplicate(
+        self,
+        db: AsyncSession,
+        user_id: str,
+        db_type: str,
+        credentials: dict,
+    ) -> Optional[DatabaseClient]:
+        """Return an existing client if it points to the same physical database."""
+        identity_keys = _CONNECTION_IDENTITY_KEYS.get(db_type, ())
+        if not identity_keys:
+            return None
+
+        result = await db.execute(
+            select(DatabaseClient).where(
+                DatabaseClient.user_id == user_id,
+                DatabaseClient.db_type == db_type,
+            )
+        )
+        for existing in result.scalars().all():
+            decrypted = decrypt_credentials_dict(existing.credentials)
+            if all(
+                decrypted.get(k) == credentials.get(k) for k in identity_keys
+            ):
+                return existing
+        return None
+
     async def create(
         self,
         db: AsyncSession,
@@ -29,8 +66,17 @@ class DatabaseClientService:
     ) -> DatabaseClient:
         """Register a new database client connection.
 
+        If a connection to the same physical database already exists for this
+        user, the existing record is returned instead of creating a duplicate.
         Credentials are encrypted before being stored.
         """
+        existing = await self._find_duplicate(db, user_id, db_type, credentials)
+        if existing:
+            logger.info(
+                f"Duplicate connection detected, returning existing client {existing.id}"
+            )
+            return existing
+
         client = DatabaseClient(
             id=str(uuid.uuid4()),
             user_id=user_id,