Everything was working well after the first install of the 6.5.1 update. Then a couple of days ago, “Duplicate Tracks” under the Project menu is greyed out! Anyone with knowledge of this and how to correct? Thanks in advance. 
I know this is literally a decade old post, but this is for in-case somebody else finds this question. I had the same issue with “Duplicate Tracks” option greyed out and it turns out I had reached the instrument track limit. Perhaps that might be whomever is reading this, issue.
Actually quite interesting. Was there already a Pro/Artist/Elements in the v6 times?
How many instrument tracks did you have? How is the limit today with Cubase 11 or 12 (Pro)?
I don’t know about Pro/Artist/Elements during V6 as I started using Cubase during 9.5.
I’m still running on that same version, 9.5 Elements I got it whilst in college. I’m looking to upgrade to 12 Pro. I believe I had 16 instrument tracks. Looking at the features of Cubase 12 Pro it says you can have unlimited instrument tracks.