datastore/query.go
func (q *Query) Distinct() *Query {
	q = q.clone()
	q.distinct = true
	return q
}
func (q *Query) Distinct() *Query {
	q = q.clone()
	q.distinct = true
	return q
}
// Distinct returns a derivative query that yields de-duplicated entities with
// respect to the set of projected fields. It is only used for projection
// queries. Distinct cannot be used with DistinctOn.