Hi,
I can now see that you have the same URL for all three instances of the "Opvouwbare waskom Defensie" product on the
www.extremesurvival.nl/sitemap-extreme-survival
page. If Xmap doesn't keep only one instance of the link on that page while it does on the XML it can only be a problem of xmap which is not consistent between both displays as HikaShop sends the same thing to xmap in both cases.
The links show several times because you have menu items configured in a way that the product can be accessed via several menu items and apparently xmap does display the link for each one on the HTML page but not in the XML page.
What you can try is to add the line:
static $already = array(); if(!empty($already[$product->product_id])){continue;}else{$already[$product->product_id]=$product->product_id;}
after each line:
foreach($products as $product) {
in the xmap plugin file plugins/xmap/hikashop/hikashop.php
And that will avoid HikaShop giving all the possible links to xmap.