[javascript] How to prevent form from submitting multiple times from client side?

On the client side, you should disable the submit button once the form is submitted with javascript code like as the method provided by @vanstee and @chaos.

But there is a problem for network lag or javascript-disabled situation where you shouldn't rely on the JS to prevent this from happening.

So, on the server-side, you should check the repeated submission from the same clients and omit the repeated one which seems a false attempt from the user.