Recently I created one different section in my phpbb forum and shifted some of the discussions to this new section. I left a copy in the original position. But now I am worried that I am not penalizing my site from duplicate content issue of google. Now definitely there is a duplicate copy of some pages as I left original copy in old place. I can’t add any redirection also as they are forum pages and one common file is used.

Should I delete the old locations and suffer page not found for some days ?