engine: fix range query duplicate label handling

[MichaHoffmann]

Prometheus seems to care about duplicate samples for label only during the same timestamp. Since our operators might return samples for one result in two different series we need to merge them to one result again.

[fpetkovski]

I would recommend that we do the mapping between different series IDs in the dedup checking operator. This way two series with the same ID would get mapped to a single ID and we will not break the operator contract.