Ingesters should ignore data with the same timestamp and contents as previously received line #1517
Description
In Cortex, if a value for a given timestamp is received twice, it is silently ignored.
Loki has no such code at the moment; as long as the new timestamp is greater than or equal to the current max timestamp, the log entry is accepted. This can lead to duplicate log lines in situations where the client needs to retry a request; for example, when a subset of the ingesters 500 on a write request, causing quorum to not be reached and causing the distributor to propagate that 500 error back to the client.
A fix for this will also be more necessary when the incremental chunk transfers code finds its way into Loki; 500 errors returned to the client will be more common as the distributor is pushing data to an ingester which is currently blocking a subset of writes.