PyMongo create unique index with 2 or more fields
You need to create a compound index and set unique to True as mentioned in the documentation:
If you use the unique constraint on a compound index, then MongoDB will enforce uniqueness on the combination of values rather than the individual value for any or all values of the key.
self.db[self.mongo_collection].create_index( [("url", pymongo.DESCENDING), ("category", pymongo.ASCENDING)], unique=True)