[Kapil]

well you arent checking if you are injecting a person, who is already injected, i.e creating a timer for a person who already has a timer. You will have to kill the time if it exist first, and then create a timer for him.
like i have done in this..
http://notesbin.com/1195081941

this should be the solution for your problems, but also makesure you are killing the timer onplayerdisconnect.