We've all been there. It's also not always your fault. Sometimes, it is delay in WGT server responding, so we click again and then have 2 of them.
Answer depends:
if not brackets: only solution is to call WGT member services and ask them to delete one. They will usually do so within 24 hours (except on weekends). And they might refuse if a bunch of members have played.
if brackets: Club owner on his own can delete as long as nobody as played a round.