When fetching the posts of a page, the response from facebook contains the paging links "next" and "previous". Those will have a "until" or "since" parameter and additionally a parameter "__paging_token"
The PagingParameters implementation ignores the paging token, but with only "until" the last post from the previous page is returned as first item of the next page.
Until seems to be the time of the last post on the previous page.
And of course there might be more than one post at this time, so facebook returns all posts starting with this time. The paging token seems to add additional information for facebook so that the next page will start really at the next post.
facebook.feedOperations().getPosts(facebookPageId, new PagingParameters(10, 0, null, null));
Example next link returned from facebook: