Duplicate meta descriptions are only a problem if they appear in multiple different types of content. For instance, if Thread A and Thread B both have the same description, this means that people that find these two pieces of content in a search engine have no way of telling them apart except for the title.
Page 1 and Page 2 of the same thread having duplicate descriptions is 100% fine, since (especially with the custom meta descriptions feature) the description will always be related to the content itself.