Can mammary glands, if only invented once, jump from branch to branch or go down the tree?
I misunderstood your earlier question. Once "invented" (your term) such a feature could be inherited by the posterity of the creature possessing such a mutation. I thought you were asking if mammary glands could also appear elsewhere on the tree, thus my response about the raw material which gave rise to the original mutation. If it still exists in the "parent" stock, I suppose the same (or perhaps similar) mutation could pop up again, in a new branch (thus my platypus comment). As you say, this seems to happen with eyes.