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I made a classic error of having WordPress show "?=" in the URL for individual pages instead of something like "/archive/title of article"
Is it worth making the change to the friendlier URL now or should I just forget it and leave it with the "?=" URL?
I picked up a WP site last year that was setup that way and switched it to permalinks. Months later and all PR is back on pages with no problems being found.
Personally, I'd switch, but that's me.
1/ Double content -> Make sure that only index page and posts (single) are (Metatag robots "all"). Otherwise, all other pages (archives, tags, by month/by day/ by year, etc.) should have a robots metatag set to "none".
Less important but it is always to have a nice and strong robots.txt which ban access to directories to bots (like photo album, wp_admin, etc.)
So, when everything is set (use of permalinks for nice URL and use of robots metatag for SEO), then I suggest you go to the Google Webmastertools, place a sitemap.xml and remove old URL.
Good luck
Tomda