Deduplicate milestone results in issue search bar filter

Milestone results show up twice when searching https://gitlab.com/dashboard/issues

To reproduce:

1. go to https://gitlab.com/dashboard/issues
2. filter by milestone is... and type "16.11" in the search for milestone

Expected: only one 16.11 result shows up

Actual: 3 (or more?) of them show up. And they are not grouped together.