In a replica-set with 2 data nodes an one arbiter
I do upserts in a collection having a unique index on a field
all works as expected on primary, 2nd upsert on same index value replaces the first one. But rs.status() reports dup key on secondary.

doing a full resync of secondary did'nt help: error reappears with next upsert.

As a workaround, I added remove before upsert and have no longer sync errors.