Replay the bug:
Enter the game => select 'Multiplayer' => (Suppose there already exist some servers), Double/Triple click one of it. You may also click quickly even for 10 times before "logging in" shows.

Then, after you log into the server, there may "be XXX(your username) has left the game"(for n times) showing on the lower left coner, n = clicks -1

And the server log:
lost connection: TextComponent{text='You logged in from another location', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}

How ever, you are actually still in the server, can go around. but can't modify anything on the server end