There are many ways to gain control of a mindless (or even intelligent) undead, but none of the methods indicate what happens when more than one have influence over it.
Lets take a good and evil cleric. The evil cleric has created an undead and has influence over it from this. The good cleric then exerts their influence over the undead. Who has control in this case? Does the source matter? Lets make it even more complicated now and a friend of the good cleric walks in, an arcane spell caster who also casts a spell to influence the undead. Now who has control?
Possible methods of control or influence are: