Diablo – Why did two Black Mushrooms spawn in the game

From the point of view of an attacker who wants your stuff, impersonating another friend you have seems like a relatively difficult thing to pull off.

1. You can only change your BattleTag once, so someone can't endlessly cycle their name looking for easy marks. A level 1 asking for high-level gear is a bit suspicious; investing a great deal of time to get to 50-60 for a con that might work a handful of times is probably a lost cause.
   
   
2. You either need to
   • accept a friend request out of the blue from someone you do not know or feigns it "Hey cute stuff, I didn't know you played D3 ;-)"
   • a "duplicate" friend request; hopefully seeing two identical names in your list would raise flags
   • or one shortly after an "oh, I'll add you on BNet" comment likely made off BNet. If they're able to monitor the latter, then you either are
     1. screwed anyways because the keylogger on your computer has already recorded your Battle.NET user/password, or
     2. royally screwed as they're tapping your home/phone/work (or standing right behind you).
        
        
3. Once your friend, they would need to maintain some level of rapport that is consistent with what your past experiences.

If you are still worried, you could take the following steps:

1. Do not add people you do not know to your friends list. If they add you, contact them through some other means (e.g. SE chat room, IM, whatever) and tell them to put something identifiable in the message that is attached with the friend request (as an aside, I've probably rejected a half-dozen requests made because my name was on the GSE list and I didn't recognize them) or check their numeric identifier afterwards (right-click name, "View Profile"):

   

2. Use RealID as Melini mentions. This is unique and will display their real name.

3. Talk to/interact with your "friends" before blindly giving them stuff. If they're level 1, don't give them stuff for their or their "other friend's" "level 59 Witch Doctor". If you're worried about giving away level 1-20 stuff, the opportunity cost of simply talking with your "friend" is vastly higher than grinding mobs in Hell/Inferno. If they are level 50-60 somehow (dubious, see above), don't blindly give them a ton of gear. Hopefully your friend would be offering other gear in return as they happen across it.

You only get experience from the bonus for enemies that give experience as normal kills. Summoned, resurrected, or low level enemies do not give experience, and therefore would count towards a massacre bonus, but would not contribute to the experience bonus.