Two computers, itunes back up Cd's will i lose what i already had?

[WaterFallGames]

Hello i just payed for my friends computer she had itunes on her computer already. i have just backed up all my music from my old computer and wand to put them on to my new computer(friends old computer) if i use the backup Cd's i have will i lose what was already on her computer? i do not want to lose any songs since most of the Cd's that was on her's was mine any ways. any help would be nice thanks