I would be slightly concerned. After checking Ticketmasters website, it seems that they email the tickets to you as a .pdf file. There is nothing to stop someone printing of 25 copies of each ticket and selling them (other than the fact they might get arrested for fraud!)
There is a question on their website regarding multiple copies and they say that only the first scan of the barcode will work. So if someone did print off and sell 25 tickets, the first person to get there wins!
What if I print more than one copy of my ticket or a photocopy is made?
A. Only the first scan of the barcode on your ticket will be allowed entry.
Make sure you keep it in a safe place like you would cash and protect it just like you would any other ticket! If more than one copy were to arrive, the barcode scanner will alert the attendant that the ticket has already been scanned. If there is a dispute, the purchaser's name appears on each ticket.