Turn off the hop up all the way down and shoot the gun without the magazine in it.
If something shoots out, problem is usually resolved. If not, try loading half the capacity of BB's into the magazine and see if the problem is fixed.
Are you using 0.12g bb's? This is a common issue with lightweight BB's. The spring in the magazine is too strong and will usually double-feed.
Are you using seamless BB's (perfectly round and not rough)? Non-seamless bb's tend to jam frequently.
And last but not least, how does he know the gun is feeding more than one? I give you the benefit of the doubt, but the gun might be feeding properly, but he thinks it's multi-feeding... it may just be jamming.
If none of our suggestions work, report back here please.