db={token:[{"token_id":1},{"token_id":2},{"token_id":2},{"token_id":3},{"token_id":4},{"token_id":5},{"token_id":5}]}

db.token.aggregate([{$group:{_id:"$token_id",/** Get the first document for each unique field*/doc:{$first:"$$ROOT"}}},{$count:"total"}])