If we use the collMod "unique" parameter, which currently has to be set to true, on a index that is already unique, the primary still generates the following oplog entry that will cause oplog application on a secondary to fail:

{collMod: 'mycoll', index: {name: 'myidx', unique: false}}

For a no-op "unique" collMod request, we should apply a strategy similar to that implemented in SERVER-48047.

This is follow-up work to SERVER-61980 where we addressed a gap in the no-op "hidden" parameter when "unique" is also provided.

This applies to a flag-guarded feature that is still under development.