I had a similar problem with double posting (it was a post not a topic ) I edited it and it was deleted.
In the future , if it happens again simply edit it and state that you want it to be deleted and a member of the Asgardian staff would do if for you.
Where I found it out to get the double post deleted
And my enquiry to double posting-
I hope these links will help you or anybody if it happens again.