(TL;DR: "App-squatters" might publish your app in Google Play, so publish them first if you ever plan to)
Over the last 6 months or so I've published several opensource apps on F-Droid. Eventually I decided I'd publish them on Google play. When I started to, I noticed a problem. Several of my apps were already on Google Play. Someone had taken the source, added ads, and published them in the store.
This is annoying, but allowed by the GPL, as long as they follow all the rules. In each case, however, they did not. None of them had links to the modified source, but still included my "about" pages which contained my name and links to my source. This gave the rather unfortunate appearance that the app that I wrote had terrible ads which pop up every few minutes.
In each case, I used the email listed in Google Play to tell them they were violating the copyright (and damaging my reputation), and asked them to please add a link to their modified source. In each case, after two days, I received no response.
So, I submitted copyright violations to Google using the tool found on this page.
For the "Existing examples of the content" section, I listed my source repository URL, the F-Droid application page, and a brief bit of text stating that the app was mine, the source was GPL-protected, the source code had been modified, and the user was failing to provide a link to the modified source, therefore my copyright was violated.
I was doubtful Google would take action, but within two days the offending apps were removed!
However, to publish my apps I still had to rename the package because Google kept the old name logged as belonging to the other user.
Also, I missed one of the non-GPL-complying apps in my copyright-violation notices, and so when I published mine yesterday, my app was flagged as a policy strike for "impersonation" because it was identical to the existing one. I've submitted a claim stating that I am the copyright holder and therefore have the right to publish, but this means that anyone who doesn't have their app already published there might be in for trouble later.