Sorry to hear your unfortunate experience.
Nevertheless, I'm not sure to catch the whole story.
1/ Did you go into different websites (other than Tripsta) to get quotations, and one of them made a pre-booking without your knowledge ?
1.2/ Which booking was cancelled ? Tripsta one or the other one ?
2/ Or did Tripsta made a mistake by booking twice ?
3/ Were you initially charged one trip or two trips ?
4/ How did Tripsta justify the 550 EUR ? Is this change fees (or whatever) ?