If the zombie used to be a creature that can be resurrected in life, it is resurrected as that creature.
The relevant quote from the description of the undead type is:
Not affected by Near death Y reincarnate Spells or abilities Resurrection Y true resurrection It can affect undead creatures. These spells turn the undead creatures into the living creatures they were before they became undead.
The resurrection spell does not directly contradict this. What it does say is:
You can resurrect someone killed by an effect of death or someone who has been turned into a dead creature and then destroyed. […] The constructions, the elementals, the strangers and the undead creatures can not be resurrected.
The first part tells you that the original creature can be resurrected if they were turned undead and destroyed, unlike Near death. The latter tells you that you can not reanimate the living dead destroyed again.
You could interpret it saying you can not throw Resurrection in undead, but then I would also be saying that you can not throw it into "living" constructions, elementals or outsiders. That clearly is not the purpose (the target is a dead creature touched); instead, he is saying that those creatures, once destroyed / murdered, can not be returned to "life".
Therefore, an undead can not be resurrected as an undead, but can be resurrected as the original creature (when available) whether it is first destroyed (the description of the spell) or not (the description of the type )
In practice, with a launch time of 10 minutes, you will not use it in a zombie that you could not easily destroy, so it's debatable anyway.