Cum at the top of condoms

Okay, so this is my first partner and the last two times we've had sex there's been either a little of him or a little of me at the base of his penis, like where the condon stops and the pubes begin. We've both cleaned up afterwards but is that normal? He's had two other partners before and he and I aren't sure if it's him or me making the mess. It's not a lot just a little and I'm not sure if any got on me so I cleaned up. But how do we prevent this? Is it a normal thing? Help.