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In Libris Mortis, there is a template Evolved Undead, gives Fast Heal 3, and says the template can be applied more then once. Does the Fast Heal go up each time?
I don't think fast healing normally stacks from the same source, but there are lots of exceptions. However, the 'evolved undead' template specifically allows it to be added to an undead creature more than once- I'd say the intent in this case is pretty clearly to allow it.
Should it stack with existing fast healing, though? Like, if the critter is a vampire? I'd let it stack but that might not be the official take on this.