Is 'مرحبًا' spelled marhabeen or marheeb?
As far as I know, 'مرحبًا' is used to address females. If we're talking to a man, we use 'مرحبا'. Both mean 'hello'.

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you can say both
(marhaban_مرحبًا)
(marhaba_مرحبا)
Both of them for men and women no difference

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Is 'مرحبًا' spelled marhabeen or marheeb?
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you are almost correct, just little small comment:
this word supposed to pronounce: "marhaban"

you are welcome anytime

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marhaban_مرحبًا) = Formal language=fuss haa .. but accepted widely

(marhaba_مرحبا)= light version of that , commonly used.

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we use  'مرحبًا' for both men and women and it's mean hello.
hope that will help you