You're really forcing me to delve deeply into this evolutionist's mode of thought. I shall continue, but only for a little while longer. As I said, I just wanted to play with your challenge, not to change my worldview. Anyway, if the genetic material from which mammary glands originally developed (mutated, whatever) is present in the "tree", wherever that may be, I suppose the potential exists for such material to be expressed again, although in a different species. I just don't know. Well, yes, there's the platypus, isn't there. I suppose, if the platypus isn't a separate creation of the lord, then it's a separate expression of the same mutated genetic stuff that resulted in mammals. Clumsy explanation, but probably acceptable to an evolutionist.
You're distracting yourself overly. If mammary glands arise in multiple places on the tree, you'll suspect it because more kinds of animals will have them and the glands in one kind will be substantially different from the ones in another. I mentioned eyes. All crazy sorts of animals have eyes and there are all crazy kinds of eyes. Some are obviously related to others and some aren't.
Just in general, you don't assume coincidence unless there's evidence that forces you. In the case of mammary glands, all the animals that have them are very significantly related in other ways. No reason at all to think there was more than one invention.
Can mammary glands, if only invented once, jump from branch to branch or go down the tree?