How to check the uniqueness of a user?

Question

Python

I have to plot the playing players by base in Mongo. I get information from the database like this:

heh = list(db.events.aggregate(
            [
                    		{ "$match": {"status": 'start'}},
    				{"$group": {"_id": "$eventName", "players": {"$sum": 1}}},
    				]))

but this code counts the total start of the level, and I need to get the number of unique users who started the level. Tell me how to check for repetition of the level? I get all unique users like this:
db.events.distinct("uid", {"is_buyer": 1})