What would happen if you cast an Energy Immunity (Fire), then follow it up with an Energy Immunity (Acid)? Are you now immune to both Fire & Acid for the duration of the spells, or does the second casting negate the first? If both are active, what prevents a player from buying a wand of Energy Immunity and making himself completely immune from damage from all five basic energy types every day for as long as his wand has charges?
Yes, they would stack as described.
I couldn't find this spell in the wonderful SRD (which is still a wonderful resource and can answer a lot of rules questions for you), so @JonathanHobbs and @BESW dug up a link to the spell in Complete Arcane. The text is as follows:
This abjuration grants a creature and its equipment complete protection against damage from one of the five energy types—acid, cold, electricity, fire, or sonic.
Energy immunity absorbs only damage, so the recipient could still suffer side effects such as drowning in acid, being deafened by a sonic attack, or becoming immobilized in ice.
Note: Energy immunity overlaps protection from energy and resist energy.
So long as energy immunity is in effect, the other spells absorb no damage.
Emphasis mine, of course.
Repeat castings do in fact stack, creating a near-total immunity to energy damage (this spell notably does not protect you from force damage, positive energy damage or negative energy damage). On the one hand, this is fairly powerful. On the other hand, 6th and 7th level spellcasting isn't a joke and it does burn up spell slots.
The stacking rules in 3.5 have to do with two things and two things only unless there's a specific exception in the wording of a spell or ability - bonus type stacking, and same source stacking. Bonus types - such as dodge bonuses, inherent bonuses, or insight bonuses - differentiate the sources of a bonus to a roll. With some small amount of exception, two bonuses of the same type (such as two enhancement bonuses) do not stack if you attempt to apply them to the same value. Likewise, two bonuses from sources with the same name do not stack, nor do two abilities or spells with the same name stack - if, and only if, they're trying to add or modify the same values or abilities. Though multiple instances of Energy Immunity do in fact have the same name, they're not trying to modify the same values or abilities - each one gives immunity to a different kind of energy. Thus, they stack.
But you can't have a wand of it.
The player can't buy a wand of energy immunity for the simple reason that it's a 6th level spell for divine casters and a 7th level spell for arcane ones, both well above the limit on wands (to wit, they can only contain spells of 4th level or lower).
The rules of the game disagree with you, gents...
Same Effect with Differing Results The same spell can sometimes produce varying effects if applied to the same recipient more than once. Usually the last spell in the series trumps the others. None of the previous spells are actually removed or dispelled, but their effects become irrelevant while the final spell in the series lasts.