To login two server at the same time with two client one account occured item duplication.
I fixed by manage information if the player is online or not.
So, If same person logged in another server using same sql database, he can't join the server.

I strongly recommend you updating immediately.

You don't need to reset your database.
YOU ONLY DO THE FOLLOWING COMMAND IN MYSQL CONSOLE.
use minecraft;
alter table player add online tinyint(1) default 0 after name;