If you have a few people creating topics such as, "Help!" or "question." or for that matter, any user generated content, you are bound to have duplicate page titles.
Another thing that could trigger duplicate titles is multi-page topics in UBBT. As of right now, UBBT doesn't state the page number in the page title. Because users can customize how many topics per page they want to see, adding "Page #" to the title wont be helpful.
For reference, one of my forums that has slightly over 50,000 topics, Google WMT reports the following information -
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