I only bought one train ticket and two were deducted.

Did you perhaps buy a return ticket instead of a one-way ticket? Then you should also contact customer service about this, but might not get your money back since it can't be determined whether or not you have used, or will use, the ticket to travel both ways.

Hi MOHANAD ALTAYAR! Welcome to the NS Community. 

We cannot witdraw money from your bank account for the exact same ticket. The system does not allow this to happen. If you have two payment, then you also have two ordernumbers that are stated in the description of your payment. 

If you have paid twice, then it is most likely that you've made the purchase two times. Did you perhaps order the same e-ticket a second time because you did not receive the e-ticket in time for your travel? It can take up to 30 minutes before you receive a confirmation about the order. It is not recommended to buy a duplicate, if you think something went wrong with the order. If you think that something went wrong, please contact the Customer Service. Because of the terms and conditions of the e-tickets, it is not recommended to buy a duplicate. The terms and conditions state that an e-ticket cannot be changed, cancelled or refunded. 

Could this be related? 

It could be that the topic starter of the topic that you're referring to has the same issue Robert B. Unfortunately, a few people decide to finish the order process multiple times instead of contacting Customer Service. I would always recommend to contact Customer Service after a payment has been succesfull and you have not received the confirmation after 30 minutes. 

Is this explained on the order confirmation page?

We cannot witdraw money from your bank account for the exact same ticket. The system does not allow this to happen. 

Interesting.