legitimacy of Twickets tickets

Gloriously, I've at last obtained a a ticket for this year from Twickets. I'm just slightly paranoid about legitimacy. Might it not be possible for people to 'clone' tickets as pdfs and sell them spuriously? Woulnd't fancy turning up and being turned away because that ticket's already been used. It seems very unlikely, but I just wonder if there's a way to check?

Would be great to hear thoughts on this