Solution #1 - hope for the best!

How can we solve the problem? The first solution may seem like a joke: do nothing, tell the user not to click twice, and hope for the best! Your page might look as Figure 2.1.

Figure 2.1. An actual screenshot of a page, just warning you against clicking more than once

This is a weasel way of avoiding the problem, but I've seen several websites that just warn the user about the risks of clicking more than once (see Figure 2.1) and actually do nothing to prevent the situation... the user got billed twice? we warned them... it's their fault! Your solution might simply look as the following code.

<button id="billButton" onclick="billTheUser(some, sales, data)">Bill me</button>
<b>WARNING: PRESS ONLY ONCE, DO NOT PRESS AGAIN!!</b>

OK, so this isn't actually a solution; let's move on to more serious proposals...