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Two greeting phrases confuse Arabic language learners!
مرحبًا and السلام عليكم
Marhaban and AL-Salamu Al-icom
The first one = Hello
The response is the same: Marhaban.
The second = Peace be upon you (This is how Muslims greet each others)
The response is: و عليكم السلام wa alicom al-salam (peace be upon you, too)
Both are used and both are correct