How many users are still double-clicking when they visit a website? It is not really a problem with links, but it could be with buttons.
Let’s take, for example, a ‘contact us’ form and a submit button which will send the content of the form by email to the website administrator. What happens if a user double-click on the submit button? The form will be submitted twice and in consequence two emails will be sent instead of one.

There are multiple ways to fix this problem. The solution I will explain below is to disable the submit button once it has been pressed. In that way, even if the user double-clicks on it, the form will be submitted only once! 🙂

To disable the submit button once it has been pressed, you simply need to add the following JavaScript code in the onclick attribute of your button tag:

this.disabled=true;this.form.submit();

For example:

<input name="submitButton" id="submitButton" type="submit" value="Submit" onclick="this.disabled=true;this.form.submit();" />

The code above will work fine if you don’t have an onsubmit attribute in your form tag. But, if you have one, you will notice that the function this.form.submit() will not execute the content of the onsubmit attribute. In that case, you need to force the call as shown below:

this.disabled=true;if(this.form.onsubmit()){this.form.submit();}else{this.disabled=false;}

However, it is recommended to move whatever you have in the onsubmit attribute in the button onclick, such as:

this.disabled=true;if(validateForm()){this.form.submit();}else{this.disabled=false;}

NOTE:
If in your code, you are using the following to check if the form has been submitted:

if (isset($_POST['submitButton'])) { echo "Form submitted!"; }

It will need to be replaced by the following:

if ($_SERVER['REQUEST_METHOD'] == "POST") { echo "Form submitted!"; }