CollMod today undergoes the following steps in order to convert an index to unique: true:

• It first sends a dryRun request to all owning participant shards
• Then it sends a collMod request to all participant shards (owning and not owning)

This operation will fail if there is a duplicate value on a currently non-owning shard since the dry-run will succeed on the first stage but fail on the second, leading to potentially inconsistent collection options across shards.