I want to see "Average Time on Site" by page.
Which item is proper to break down each page?
If I use "URL" segment like below, URL is both including original URL and tagged URL, so there's duplicate.
I would really appreciate if you can help me.
By "Original URL" and "Tagged URL" (since I can see you blurred your data for privacy) I am assuming that you are talking about query string parameters?
Which is causing the URL to have multiple variants
You could try use Page (which the name should be consistent, unless your page name changes), or you could try creating a classification on your URL dimension to remove all query strings, so that the variations will collapse together into one row? Or you could set up a separate eVar for tracking something like "Canonical URL" (if you have multiple sites sharing content, and your SEO Canonicals are set up correctly, this would also allow content that is shared across multiple sites to collapse into a single row - so that you can get the average time across all sites)
By this I mean:
siteA.com/pageX (canonical is siteA.com/pageX)
siteA.com/pageX?cid=123 (canonical is siteA.com/pageX)
siteB.com/pageX (canonical is siteA.com/pageX)
siteC.com/pageX?cid=635 (canonical is siteA.com/pageX)
By tracking the Canonical URL, all 4 URL variants across 3 sites would be recorded as "siteA.com/pageX"