I debated which forum this should go in, here or alternative play, but I decided here was most appropriate because I want an official answer and not a house rule or a variant. If I made a mistake, I apologize.
So, my question would primarily come up in Team games. I'm having a hard time thinking of another situation in which it might arise. But, if you are on a team where both people are playing the same mage, can you stack the same ability twice on the same object?
For Example: You have two Beastmasters on a team (both of them the original Straywood Beastmaster). Could they both make the same creature their Pet? The rules say "The Pet marker can be assigned to any new friendly animal when it comes into play, regardless as to how it was summoned." So it is very clear that they would be allowed to make a animal summoned by their teammate into a Pet. However, what happens if they both want to make it into a Pet?
Enchantments, Equipment, and Conjurations do not stack. Conditions, however, do stack. What about Mage Abilities?