problem with project

When the user hits the button for first time set some state somewhere i.e. have some specific value in hidden variable or viewstate or set some session variable.

then when the user hit again first check this variable and if it has been set don't do anything just show some message.

which state management you resort to depends on various project specific things so that your choice.

UPDATE:

But this will only work at most in the current session. TO make this behavior persistent for a particular user you need some sort of mapping table which will take care of tracking such things.

so if your DB there is something like a JobPostID, and a USERID. you could also have a ApplicationTracer table which will contain TrackerID, USERID, JobPostID.

so when ever user apply for a job you can check whether that job ID contains this UserID in the tracking table and let him apply. once he apply make entry in this tracking table to reflect the same.

Yes i completely agree with Sandeep Mewara.
To prevent seekers from applying on same job you need to keep track of applied jobs of job seekers.

The Better thing is that when anyone user can applied for job, then store Jobid and Userid in database. And when this user will try to apply for the same job check the function if user already apply then pop up message.
It just like the Duplication record checking.



Thank You.