All vitamins (to the best of my recollection) excrete through breastmilk. Sort of necessary, actually. Or all of our babies would die from malnutrition. They need the same vitamins and minerals we do. Just in different dosages.
The best prenatal you can take you won't find in a bottle... but on your plate.
We readily absorb vitamins and minerals from the foods we eat, but studies show that the vast majority of pill/liquid vitamins end up in our toilets... peed or excreted out because our bodies just can't/ don't absorb them.
So the best thing to do is to change your diet so that you're eating everything you need. Of course, that's expensive and time consuming and most of us can't do it... because you not only have to buy a lot of fresh foods... but also cook them correctly (most vitamins and minerals are volatile... they break down with heat... but others only become absorbable with heat and are useless in raw foods... so it can be a bit of a pain in the neck to research what's what.)
Since you're already taking supplements, you have added research, since you need to find out what the dangers are from double dosing. Any that are water soluble, no problemo... they just get peed out (as long as you have healthy liver and kidney function).
My *suspicion* is that your dietician already did that research for you. Give him/her a call to double check the ICAR v ICAR-C. They should be able to tell very easily whether it's just the same thing with a notation, or a totally different thing from what they recommended.