The main SEO danger is that the search engines don't like to display duplicate content, and are likely to filter all but one version of a page. In Google, this is generally the page with the highest PageRank. The problem is you have no control over which url that will be.

While there are various ways of blocking the page so it's not indexed, it's possible that someone might link to a blocked page and the link will be wasted.

When you say "several sites with different urls," I assume btw that you mean several sites with different domains.

Here's some discussion on another thread of sites with similar considerations....

2 Sites - Duplicate Title Tags - Loss of Rankings for 1
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