I like
Mercy but worry that a little girl would constantly get
Marcy (which I do not like). I'll be the odd one out and say that
Mercy Marie is my favorite combo (fn/mn with same letter is a secret love of mine...) The flow is very nice and I say if
Marie is meaningful, use it!
Marie is a family name for me as well and so despite its use as a "filler mn" I'll likely use it.
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Betsy-----