Duplicate locations displaying in External Career Site

Summary:

Duplicate locations are displaying in external career site under the Locations filter field:

As you can see from above screenshot, Glasgow and Dundee are displayed twice (no, there are no duplicate primary locations).

When the candidate selects the Glasgow with 6 options in brackets, 6 jobs are displayed and there are no longer duplicates in the Locations drop-down:

If I then clear filters, and click on the Glasgow with 3 in brackets, the exact same list is displayed, i.e. 6.

When I select the Dundee options, one at a time, only 1 job displays, never 6.

There is something related to additional primary locations but I am not quite understanding the behaviour. Anyone seen this before?