Controlling undead

I am wondering what happens when a wizard uses the spell control undead on a zombie and then a evil cleric tries to use his turning ability on that same creature.

The spell description implies that it can't be rebuked or turned or whatnot because it's not using it's normal behavior when you are controlling it with the spell:

This spell enables you to command undead creatures for a short period of time. You command them by voice and they understand you, no matter what language you speak. Even if vocal communication is impossible the controlled undead do not attack you. At the end of the spell, the subjects revert to their normal behavior.

I would let the most recent effect override any previous ones (and if the latter effect has a shorter duration, the older effect comes back when it expires), so the cleric could command the zombie, just as he could steal it from a rival cleric. Of course, the wizard could then cast his spell again to regain control, etc... If you want, it could be handled as an opposed charisma check each round to see who the zombie obeys.

The cleric should get something if he's powerful enough to command the zombie, though. At least the lesser result of rebuking it without taking control of it. If the evil cleric can't even rebuke a wizard-controlled zombie, why can a good cleric turn it?

EDIT: Looking closer, while I still think the zombie can be turned/destroyed/rebuked/controlled by a cleric, I could see an argument that even if the cleric controls the zombie, it cannot harm the wizard for the spell's duration.
http://www.d20srd.org/srd/spells/controlUndead.htm

So any other thoughts? So far there is one vote for the wizard keeping control and one for the cleric getting control of the undead, with the possablity of it not being able to attack the wizard.

I see it as a fight for mental control resolved by opposed charisma checks when conflicting directions are given.

srd said:

Multiple Mental Control Effects
Sometimes magical effects that establish mental control render each other irrelevant, such as a spell that removes the subjects ability to act. Mental controls that don’t remove the recipient’s ability to act usually do not interfere with each other. If a creature is under the mental control of two or more creatures, it tends to obey each to the best of its ability, and to the extent of the control each effect allows. If the controlled creature receives conflicting orders simultaneously, the competing controllers must make opposed Charisma checks to determine which one the creature obeys.

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