Multiple Confirmation Emails

It means that there was an error on the notification page triggered by paypal on your website and thus paypal thought that it didn't work. Thus, it sent the message again and again until it stopped for good.
It's impossible to say what is the problem without looking at the notification page on your website. Could you give a link ?

No, what I need is the URL of your shop so that I can order something and see what paypal is seeing for that order.

The notification page is working ok up until the IP check which fails since we're not paypal.
We would need an FTP access in order to put traces in the code and try to understand the problem.

Sure. You can use our contact form: www.hikashop.com/en/support/contact-us.html

Thank you. We'll look at that today.

I couldn't find the problem as there is no error message as all. Everything seems to work fine during the notification process so I don't know why paypal tries to resend the notification several times.

In order to circumvent the problem on your website, I've added the code:
if(in_array($dbOrder->order_status,array('confirmed','shipped'))) return true;

after the code:
if(empty($dbOrder)){
echo "Could not load any order for your notification ".@$vars;
return false;
}

in the file plugins/hikashoppayment/paypal/paypal.php

That will avoid that the process is done several times but I'm afraid that you will have to reapply that modification when you update hikashop in the future...