I’ve got a demo domain for some websites that I’m putting together - each site will go in its own directory…
www dot demowebsite.com/site1
www dot demowebsite.com/site2 etc.
Once a site is ready to go live I’ll delete it from the demo domain and put it live on its own domain.
My question is: will this give me duplicate content issues with Google?
If the answer is yes… can I simply de-index the domain and block all robots with robots.txt to prevent and problems?
You can make a simple 301 content redirection from old domain to new, without any problems with Google…
Sorry, I’m bit late with this reply, but here it is anyway. 
If I’ve understood you correctly, the demo site is not really for public consumption - just for you and your clients to view. So you don’t want that one indexed at all. The easiest thing is to permanently block all demo sites via robots.txt. That’s the way I do it, anyway.
Thanks for the replies - I ended up going with the robots.txt solution which has worked well!