will be detected as double spend?
Yes, just wait for a couple of confirmations.
will he knows that he pays for 1 person?
I'm not sure what your asking here. Do you mean can he only pay one person with the same coins (and not double spend)? If so the answer is yes.
will transaction will be safe for me? i mean buyer cant cancel the transaction
Yes it's safe, once you have the coins the buyer cannot reverse the transaction.