This has been found to occur when looking at the UPM page in Admin, however any location retrieving the users from a group that has more than 100 users could trigger the error.

It is related to how Crowd, the library used by Stash and other Atlassian products to manage users, would return duplicated users in some circumstances (CWD-3628), while Stash would not expect any duplicates.

It is not a symptom of corrupted user data.