implement duplicates=no for the api [Probeaufgabe]

[Stephan Doerfel]

This is relevant for our extensions when a user uses custom-URLs to fetch. See the related item https://bitbucket.org/bibsonomy/bibsonomy-typo3-csl/issue/10/publication-list-contains-duplicates

[Robert Jäschke]

Can we push the existing code from the webapp down to the DBLogic? Then this would be available for both the webapp and the API.