When posting a YouTube vid DO NOT EMBED etc. Just simply past the URL (http://www.youtube.com/
....) into your posts. Not the EMBED code they provide.
With Vimeo clips simply click there Share Link button and then the Copy URL button and then paste that into your LGM post/thread.
Any other methods will cause that ugly duplication thing and We do not have time to clean all that junk up so thanks in advance for making life easier on the board(s).