feat: dedup downloads by (url, format_id) index, reuse existing files

backend/app/database.py

@@ -21,3 +21,9 @@ async def get_db():
 async def init_db():
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
+        # Ensure composite index exists on already-created tables (idempotent)
+        await conn.execute(
+            __import__("sqlalchemy").text(
+                "CREATE INDEX IF NOT EXISTS ix_video_url_format_id ON videos (url, format_id)"
+            )
+        )

backend/app/models.py

@@ -1,6 +1,6 @@
 """SQLAlchemy models."""
 from datetime import datetime
-from sqlalchemy import Column, Integer, String, DateTime, BigInteger, Text
+from sqlalchemy import Column, Integer, String, DateTime, BigInteger, Text, Index
 from app.database import Base
 
 
@@ -9,7 +9,7 @@ class Video(Base):
 
     id = Column(Integer, primary_key=True, autoincrement=True)
     task_id = Column(String(64), unique=True, index=True, nullable=False)
-    url = Column(String(512), nullable=False)
+    url = Column(String(512), nullable=False, index=True)
     title = Column(String(512), default="")
     platform = Column(String(32), default="twitter")
     thumbnail = Column(String(1024), default="")
@@ -24,3 +24,7 @@ class Video(Base):
     progress = Column(Integer, default=0)  # 0-100
     created_at = Column(DateTime, default=datetime.utcnow)
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+    __table_args__ = (
+        Index("ix_video_url_format_id", "url", "format_id"),
+    )

backend/app/routes/download.py

@@ -50,6 +50,19 @@ async def _do_download(task_id: str, url: str, format_id: str):
 
 @router.post("/download", response_model=DownloadResponse)
 async def start_download(req: DownloadRequest, background_tasks: BackgroundTasks, db: AsyncSession = Depends(get_db)):
+    # Dedup: reuse existing completed download if file still on disk
+    existing = (await db.execute(
+        select(Video).where(
+            Video.url == req.url,
+            Video.format_id == req.format_id,
+            Video.status == "done",
+        ).order_by(Video.created_at.desc()).limit(1)
+    )).scalar_one_or_none()
+
+    if existing and os.path.exists(existing.file_path):
+        logger.info(f"Reusing existing download task_id={existing.task_id} for url={req.url} format={req.format_id}")
+        return DownloadResponse(task_id=existing.task_id, status="done")
+
     task_id = str(uuid.uuid4())[:8]
     video = Video(task_id=task_id, url=req.url, quality=req.quality, format_id=req.format_id, status="pending")
     db.add(video)