This isn’t anything you are doing wrong, it is a bug. Issue linked below. However, I will note that the description of the bug is perhaps incomplete based on some testing I did this morning (7.9.6.2). Even if there are no two CollectionObjects that share the same locality in the query, it may still duplicate the markers. The screenshot below is for one collection object, with both markers listed with the same id (while loading) and then catalogNumber once rendered. In a query of 45 collection objects with localities, I get 90 markers returned, the same 2:1 ratio you are experiencing.
If we look at this same location through a different frontend, we only have one record with that location, so it isn’t an issue of a record outside of the query having the same coordinates.
Hello @kitchenprinzessin,
I was able to recreate this issue and found that in GeoMap the points are double the amount of collection objects added instead of just having one point for every collection object. I created a report for this issue on GitHub: