I don't know the evolutionist's response to this. I've seen evolutionists talking about how eyes and other structures can develop numerous times, in numerous ways, in unrelated creatures (recently unrelated, anyway). Mammary glands could undergo the same (random?) re-evolution, time and again, I suppose. One wonders where creation leaves off and allegedly unguided mutation begins in such cases. But as I said, I don't know.
A few things like the eye seem to have arisen independently in more than one line. Insect eyes, for instance, are very different from ours. Mostly, this is easy to spot although there can be problems if all you have are fossils.
Here's another point you may not be considering. An innovation, mammary glands, appears in one place on the tree. Can it move down? Can it move to the side?
Where can you find it later on?
Actually, evolutionists will tell you the progenitor of the eye arose only once and the variations of eyes found throughout nature are the result of local selection. The prediction here is that the gene encoding eyes will be similar -- NOT IDENTICAL (that's so gore3000 cannot twist my words later) -- in all animals (it is) and that all animals possess this gene, even if they do not express it.