In philosophy, something is overdetermined when there are two or more causes, any of which would have been sufficient to bring about the conclusion. As Sarah Bernstein explains:
Billy and Suzy each throw a rock through a window, and either rock alone could have shattered the window. It’s a case of causal overdetermination (hereafter, just “overdetermination”) because there are multiple distinct causes, each of which are sufficient to bring about the effect.
Or, shorter and more formally (again from Bernstein):
Causes c1 and c2 causally overdetermine an effect e if c1 and c2 are distinct, they actually occur, and they are individually sufficient to cause e in the way that it occurs.