In the movies and books it seems like the armors are often very similar, and in the mayhem of the battle it would be very likely that someone would duel somebody from its own side without even noticing.
Also if that wasnt usual, what strategies did they use to prevent this kind of problem?
Not to discourage new answers, but as u/Animastryfe noted, this question was answered in an earlier thread by u/PapiriusCursor and u/duckofuork