I recently purchased a vehicle and whilst transferring ownership, the staff at the DLT have made a mistake in that they've listed my middle name twice omitting my surname from the book.

I brought the error to their attention but they insisted it isn't an issue.

Will this cause any difficulty when I come to sell?

Are you a Falang? it won't match the copy of your passport page name etc etc when you want to sell so go back and insist they change it.