What i read from google support is. You can have duplicate content on your site and your should present that duplicate content to different users and not for the same user.
One or more of the pages where it is duplicated will be devalued. So only one will rank well. You can make preferences however... Search the DP forums for "canonical tag" or have quick a look at en.wikipedia.org/wiki/Canonical_link_element Cheers
I know from an SEO perspective it's not good although it sounds like the canonical tag should help. I'm wondering if I would get banned by AdSense for doing it.
For adsense, no problem at all. For google search they even said, it's not a problem, they take care of it internally and try to show only the version of the content which they think is the original one or whatever. There will not be penalities.
I don't think there will be any problem for adsense account or earning. But at same site why do you have duplicate pages of same content?
There can be reasons for this, I know because I'm in this case right now, I forked my site to another domain and from now on the content added will be different, but the 5000+ first pages will be the same. But I'll change the template and also some texts at many places in the interface to reflect the difference between the two sites.
Does it mean your both sites will be live? In that case you can be in trouble, use redirection or canonical tag
Yes both sites will be live but will be very different. What trouble exactly? I'm not paranoid. I don't want to do a redirection and for the canonical tag there's not much reason to do it, it's optional, you can do it if you want to help googlebot decides which page to show in priority when he finds two identical content, but that's it. There's no reason for google to ban any one of the site since they will be different in the template and also in the content. Of course the first thousand pages have the same content text, Google is free to pick whichever page it wants to show, from one site or the other. But everything surrounding the content will be different, and also the sites as whole are going to be different. It's been only a few days and already the first site has more content which is not on the new one since it isn't live yet. As soon as I make it live, the new one will also have new additional content posted which will be different from the first one. I don't see what trouble I could be in and why..
Duplicating content on your site seems redundant -- it may not violate Google's TOS, but it probably won't do your site any good.
Yes, may/probably that's what I believe too, but there is the occasion to see it in reality. Reality might be different from what we believe, or not. Doesn't matter in any cases.