Duplicate landing page links on add content page

What version were you running before? Did you run update.php (or drush updb), revert features, and clear cache as described in the update instructions?

Otherwise, it sounds like you have a duplicate content type with a different machine name. Landing Page is part of Panopoly rather than Open Atrium. I haven't seen this on any other sites so it must have been something done on your local site.

The one that points to node/add/panopoly-landing-page is the correct one. Looks in your Structure->Content Types for a duplicate with the landing-page machine name. If you can verify that you don't have any content using that content type then you can remove it.

Delete the duplicate menu link. the reason why it displays the node/add page at the "duplicate" link is that there isn't a menu router item for the node/add/landing-page item so it falls back to node/add (cause that's how the menu system works; check out node/add/gibberish, node/add/whatever-is-not-a-node-type).

The node/add menu is actually made from the menu system, it shows all the menu links "under" node/add (navigation menu I believe but could be wrong). It sounds like at one point someone made a landing-page node type then removed it, and the old menu link still remains for some reason (was customized perhaps?).