My first guess is that this has something to do with the merger of Eurostar and Thalys, and perhaps some changes to their reservation system.

How many PNRs do you have? That's the alphanumeric booking reference you'll find on your reservation.

I'm guessing this is a trip to or from London?

I only have the one booking reference that is alphanumeric. There is another reference (tagged as both reservation code and PNR) but is just 6 letters and not included on the second reservation.

Yeah, it's Brussels to London.

I have tweeted SNCB to ask if there is an email I can contact them via. I won't get an answer until tomorrow, if they even do answer.

If I can't contact them it will have to wait until I am in Belgium, which won't be until about 4 days before I travel.

PayPal shows I only paid €34. I will have to just say it's their mistake.

I guess it's not as if I can even enquire in Berlin when I'm there in a few days. Would they have offices in Amsterdam?

I wouldn’t worry that much ! As long as you’ve not been charged twice I wouldn’t do anything.

Please let us know what their answer on Twitter is. :)

And I agree with ralderton the merger may have created some kind of mess...

It was suggested I send a PM to SNCB on twitter, which I can't do because I am not verified.

I sent one via Instagram instead. I hope they see it.

In their reply on twitter they suggested Eurostar may have modified my seat, but not to worry as I shouldn't be charged.

Seems a bit bizarre that an old seat would remain on my ticket on the app.

Seems like it's all a bit of a muddle to me...

I'd considered this matter closed. Some helpful ladies on twitter had given me an answer that it was probably an admin thing with Eurostar assigning new seats because there were now less coaches. This answer may help someone else, and you asked to know the answer @thibcabe

I was told the following;

You've paid only one time .The fact that you see a double ticket is when eurostar relocate a passenger meaning that they (eurostar ) assigned you a new seat in another coach . This happens when the train is running with less coaches then forseen when you've booked .

The reason why you see a double booking is because Eurostar doesn't seem to be able to remove the original seat even if that ticket is not anymore active . It's purely a technical issue .

Then today I got an email from SNCB, after I had previously contacted them via their support form. It's quite the misunderstanding of my request.

I'd told them I had a duplicate reservation. Somehow they took that to mean I wanted to exchange.

I'll be in Belgium for this train in less than a week so I'll ignore that misunderstanding for now.