It has been observed on a cluster the presence of 4 migration coordinator documents on one shard that led to hit this invariant on step-up.

The documents were all relative to migrations for different namespaces and the states were:

• 2 aborted
• 1 committed
• 1 without decision

The range deletions seemed to have been correctly handled both on donor and recipients:

• No range deletion documents for the aborted migrations (range deletion tasks already executed)
• Ready range deletion task on the donor for the committed migration
• Pending range deletions on donor/receiver for the migration without decision

Given the state of "decided" migrations, we can consider that:

• _abortMigrationOnDonorAndRecipient worked well.
• _commitMigrationOnDonorAndRecipient worked well.

It is then very likely that something odd happened right after, as part of the call to forgetMigration that did not remove the migration coordinators.