dirPagination should warn when there is duplicates

Hi,

The directive failed to work because the array I was willing to paginate included duplicates. NgRepeat warns about it while with dirPagination I had no warnings in console log.

Start by locating the dirPagination directive and reviewing how it processes the collection, then compare the requested behavior with AngularJS ngRepeat's duplicate warning. Done means duplicate items produce a console warning while normal pagination continues to work; verify behavior with the project's relevant tests or manual example.

Written by the indexing model from the issue text.