N1QL: Upsert command not working correctly

Issue an upsert query first time:
UPSERT into `beer-sample` (key , value) VALUES ("key_test",

{"test":123}

);

This works correctly, inserts a new key,value pair in bucket.

Issue an upsert again with a different or same value, with same key:
UPSERT into `beer-sample` (key , value) VALUES ("key_test",

{"test123":12345}

);

This gives error:
[

{ "code": 12009, "msg": "DML Error, possible causes include CAS mismatch or concurrent modificationFailed to perform insert - cause: Duplicate Key key_test" }

]