Right now, because of some server side issues (probably oversensitive ddos protection), you can lag out randomly at any time. Many of you reading this have probably experienced this before, just maybe not in pvp. It usually happens every few minutes. When you lag out, you will probably be told you're already online for the next 30 seconds or so when you try to connect. Until this is fixed, I personally don't recommend pvping with anything more than prot I iron, unless you're really rich. I myself just lost a full set of Prot IV Thorns II, which inspired this post.
This only happens to me when I go AFK in front of my skele spawner for 4 hours. A way to bypass the "already online" thing is to restart mc OR to join a SP server, then go back to MP.
That doesn't fix the problem, it just works because it takes the same amount of time as waiting in the server list. It is a server side issue.
Woops XD. Still almost definitely will get you killed in pvp. Also, it probably happens most at the skele farm because that is where most time is spent. If 10% of time is spent somewhere, say spawn, on average 10% of these incidents would happen there.