Revned had mapped much of the NES Mega Man games, though some games had a few maps done by other people. When Revned revised all the NES Mega Man maps with various fixes last year, we didn't think it would be a big deal to replace the few maps done by others for consistency.
However, Ryan had mapped out Mega Man VI, and his maps have enemies, a detail which Revned's do not, which Ryan had pointed out just before I was going to put Revned's new ones up. Revned stated he wasn't out to invalidate others' hard work and supported that Ryan's be kept, so they are, but it does bring to light issues about if someone makes a "better map". And then who's to say which is better - in this case Ryan's includes enemies and Revned's is more consistent - pixel-wise, palette-wise, etc. with the other five games.
But if someone were to take someone else's existing map and just add enemies and items, and the map is clearly better, but they didn't do the initial mapping, then what? You can't take someone else's movie and film a few extra scenes and claim the movie as your own, right? So while no one profits here, it's still not very nice. But how can you prove it either way? If you're mapping the same game, and you're both doing it correctly, even if you legitimately made the map from scratch, how would we know? And we could also keep both, but then it would be redundant. Thankfully webspace and bandwidth are both dirt cheap since I'm now with GoDaddy, but there's still the sheer clutter of having more maps than necessary. Surely you wouldn't want me to accept any and every map that comes my way?
Generally I use common sense, and the ones I've rejected and/or replaced in particular cases haven't caused too much backlash. Though I'm not perfect and I wouldn't be surprised if someone out there is bitter about my choices.
But unless you have a good idea for a viable "system" as you suggest, Ryan, I'm not sure this will ever be easy.
What would be ideal is if the first person who maps a game does it perfectly, and that people don't ever plan to map the same game. Discussing techniques and "calling dibs" on the forums helps in both cases, but only with the regulars here. As snesmaster/Rick Bruns points out, he maps games for himself and his own web site. Unfortunately, VGMaps.com isn't THE screenshot maps place (even if it is the biggest, it's still not very well known overall) so duplicity is bound to happen since very few people in the world even know that some of us are calling dibs on mapping certain games on these forums when they don't even know of these forums or of this site for that matter.
I will have to continue to have to judge things on a case-by-case basis, but I will consider any suggestions of alternative solutions or systems that would make future decisions easier to make.