[{"field1":{"$numberDecimal":"0.99"},"field2":0.99}]

db.collection.aggregate([{"$set":{field1Normalized:{$cond:{if:{$isNumber:"$field1"},then:{$toDecimal:"$field1"},else:"$field1"}},field2Normalized:{$cond:{if:{$isNumber:"$field2"},then:{$toDecimal:"$field2"},else:"$field2"}}}},{"$match":{"$expr":{"$eq":["$field1Normalized","$field2Normalized"]}}},{$unset:["field1Normalized","field2Normalized"]}])