Hallo @Meerrettich, there are certain restriction for merging two places: The pin must be on the same position, category and name must be the same… etc. It also plays a role, if somebody already claimed ownership to one of the listings.
Have a look at @MortenCopenhagen’s article : #LGCTM Tips & Tricks: Merge Workflow
Unfortunately this happens often when users don’t check carefully, if it already exists before adding a new place to Maps.