Unspoken assumptions wrapped up in this question:
That the God in question has the ability and the inclination to watch every moment of Their followers' lives.
That the God in question would not allow two followers to fight using The powers they have been granted.
That the believers in that God have any right to question the criteria by which the God grants power.
Without those assumptions, the original question is no longer able to be asked. The Warhammer Gods are not omnipotent, omniscient, omnibenevolent, or infallible. Sometimes they aren't paying attention. Sometimes they don't care. Sometimes they're too busy dealing with something else, or too expended by doing so, to intervene in a given situation. Sometimes they change their mind, sometimes they're tricked, sometimes they're charmed or amused. Inconsistency on their part does not undermine the whole institution because nobody in universe (except a few monotheistic heresies) is expecting perfect consistency of them.