f-droid keeps downloading of apps that were installed from other stores f-droid again and again downloads updates for a app that is installed via another (namely play) store.
Obviously it cannot be installed because of different signatures.

How can i get F-droid to ignore foreign-store apps.
For these common apps-  In F-Droid, open the info page of an app in question, click on the 3-pointed menu and select to ignore all updates for this app. 



Source

Edit: Thanks to input from BubullC this feature is incorporated in the latest version of F-droid. I haven't verified it but adding it for benefit of future vistors