Hi,
No You cannot overfilter as such... you will find though that you get to a point where aditional filtration is not as beneficial as the bacteria colonies in the filters will only grow according to how much food (ammonia and Nitrite) they receive - So whereas one filter maybe fully colonised with bacteria, if you had 2 or 3 on the same tank, you would find they maybe only half colonised with bacteria each.
Of course, the other aditional benefit to multiple filters is there is more fine media to catch debris / particals in the water...
Aditional Filters also mean aditional flow of water, so again it depends what type of fish you have whether that is suitable or not..
Regards,
Craig