iDeal: Request with fnc=handleMollieReturn param is called twice sometimes

[JaroslavHerber]

Tested versions: 1.0.32 and 1.0.33
We use the iDeal Payment-API.

This issue is not reproducible in test-mode.
In live-mode we see such double requests (in apache logs) in about every 2nd "successful" order from real customers.

Here is an example of such a request, that is fired into the shop two times at exact same time:
/index.php?cl=order&fnc=handleMollieReturn&stoken=1234&sDeliveryAddressMD5=example&force_sid=abcde&ord_agb=1&rtoken=xyz

One of them has a referrer "https://www.mollie.com/". The other has no referrer. All other params are same in both requests.
Both requests are also fired from the same customer-IP.

The module redirects those requests to the thankyou-controller if the order was successful. But OXID allows this call only once. The second call of this controller is redirected to the start-page.