I go "oops I probably should have realised that existed before I decided to purchase the rights to and re-release a 999999 year old game which already has a GPL clone/spiritual successor/something".

I then go "well why re-release this ancient game running in an emulator, when this exists?" and ask the core team of OpenTTD if they want to monetize their steam/GOG releases now that I can licence out the TTD IP to them and remove any remaining legal ambiguity (and recoup my """investment""" via revenue sharing).

And if they don't I take it as a learning experience (to do my homework before I buy IPs) and release my TTD-in-an-emulator on steam and GOG knowing full well that its probably not going to generate many sales. Maybe I add "hey just so you know there's this really cool modern source port you can get for free..." to the description and hope that I can generate some sales off of good boy points.