I need help on borderlands...PLEASE HELP ME!!!!!!!?

I just got borderlands, and I played for 45 minutes and am a level 4. I was running around picking up all the dropped ammo and money. I wasnt even looking at the ground but I accidentily picked up P pistol and lost my combat rifle. i went to this vending machine, It gave me the option to sell the gun but not equip it. By the way i now have two pistols.