Multiple "base" entries on the right dropdown in MR diffs

Summary

Multiple "base" commits show up on the right hand dropdown in the MR diff view.

Steps to reproduce

Not sure how this happens exactly, I just observed it. When master is selected, both entries have a checkmark too.

Expected behavior

No duplicate entries.

Actual behavior

Duplicate base entries.

Relevant logs and/or screenshots

gitlab-duplicate-master

Output of checks

No access to the installation's shell.

Possible fixes

Dedupe the list before rendering?