Multiple form submits is a serious issue in web applications because it’ll result in unexpected behavior like multiple entries in database .I have spent some time for the same in Google and I got some solutions for that.
If you are submitting the page only once then you can use,
<form onsubmit="return Submit();">And the method is like,
For a single button you can use,
For pages with update panels multiple submit is a serious issue as page is posting asynchronously or partially.In that scenario you can use Sys.WebForms.PageRequestManager for fixing the issue,