I’m not familiar with the app, but I looked a few things up and I can point out a couple of things that look incorrect in what you stated:
- It’s the one by Mark Bestravos that was a fork, and the one by wangdaye is the original version, as stated in the app description of the former (it was created to get rid of some non-free dependencies, but now the changes needed to do that have been moved into the “upstream”, i.e. the original, version, and so there is no need for it any longer)
- The apps do have two different IDs: the original one by wangdaye is wangdaye.com.geometricweather, while the fork by Mark Bestravos is com.mbestavros.geometricweather (it’s true that they have the same name)
- The reason for this latest 2.610-foss release of the one by Mark Bestravos is explained in the release notes: “The primary change with this release is a pop-up notification on launch that notifies users of the above change and recommends they download the newer app for continued updates”.
So, ideally you wouldn’t have the Mark Bestravos fork installed anymore in the first place, and only the app by wangdaye; the final version of the Mark Bestavros fork exists precisely in case you still have it installed, to inform you that you are supposed to switch. Now, why you’re informed that there is an update available to the Mark Bestravos fork when, in fact, you said you removed it, I don’t really know: the fact that they have different IDs is normally sufficient for F-Droid to consider them separate apps (even with the same human-readable name), and it really shouldn’t propose any update to 2.610-foss. Are you quite sure you don’t have both still installed?