We all do it at sometime or another! If you put up a duplicate post and realise you've done so it within the hour or two allowed for editing, simply delete everything and put "Duplicate post" or similar.
If you realise you've posted in the wrong section in the same sort of time-scale, cut and paste the text into a new post in the correct section, and label your old message with something like 'Wrong part of forum - moved to XX section'
Regards,
John Webb