Add unique constraints to the task_id and taskset_id columns in the database result backend.

celery/db/models.py

@@ -19,7 +19,7 @@ class Task(ResultModelBase):
     id = sa.Column(sa.Integer, sa.Sequence("task_id_sequence"),
                    primary_key=True,
                    autoincrement=True)
-    task_id = sa.Column(sa.String(255))
+    task_id = sa.Column(sa.String(255), unique=True)
     status = sa.Column(sa.String(50), default=states.PENDING)
     result = sa.Column(PickleType, nullable=True)
     date_done = sa.Column(sa.DateTime, default=datetime.now,
@@ -46,7 +46,7 @@ class TaskSet(ResultModelBase):
 
     id = sa.Column(sa.Integer, sa.Sequence("taskset_id_sequence"),
                 autoincrement=True, primary_key=True)
-    taskset_id = sa.Column(sa.String(255))
+    taskset_id = sa.Column(sa.String(255), unique=True)
     result = sa.Column(sa.PickleType, nullable=True)
     date_done = sa.Column(sa.DateTime, default=datetime.now,
                        nullable=True)