It really depends. How simular is game X to game Y? How many games are all simular to eachother (ie: stale, boring, borwn and grey marine shooters)? Does the game stand out on its own? Unfortunately for you guys, I ran into some free time, so please delight yourself by skipping my long breakdown into my opinion about game clones.
Look at Viking. It is basically God of War, but with Norse Gods instead of Greek Gods. Having little experience with God of War, I cannot judge how different gameplay is. I think Viking involves more stealth at some parts, and battles involving hundreds of AI on both sides of the fight. Did it sell good? Not really. This is evidence that it is not the game quality that hurt its sales, but rather the fact that it was another hack and slash "like God of War but" games. Even though it could have stuck out on its own merit, the genre was too full.
I'm not even going to get into shooters. I've pretty much stopped playing them. The market is so full it is practically leaking random shooters into my previously played games section.
Let's look at Saint's Row 2. Whie GTA4 went for the realistic approach, SR2 went for full over the top juvinile awesomeness, and it worked. Saint's Row 2 is just about everything I could want out of a Sandbox Crime Game. Not just a copy of older GTA games, it stuck out because it did everything better than before. Naked skydiving, spraying poo at people, throwing people into jet turbines, ragdolling yourself through the air intentionally getting hit by semis, this was not encouraged gameplay, it was included activities that gave in game rewards and achievement points. The desginers of SR2 went above and beyond, thus freeing themselves from underneath the foot of GTA, and becoming its top threat. In Saint's Row 3, there are going to be lucha libre wrestlers and a pimp which says everything in autotune. I think I'm in love with the series.
What does this conclude? It doesn't affect how great the game is, but if too many people are also copying the same thing, the whole experience gets watered down, and it is harder to make a game that stands up to it's inspiration's legacy.