Last modified: 2013-01-14 15:54:07 UTC
Entering a language link to the same article through Wikidata and locally should not lead to the link being displayed twice.
Just tried it and yes, it is possible to duplicate the links. I'd suggest taking a look at and reusing the old client code before refactoring, since a number of things that were solved in the client do not work anymore. This, but also noexternalinterlang parser function, and probably more.
noexternalinterlang should work. As far as I understand the old client code was widely reused.