-
Click on the cousin.
-
Click "Add partner/ex" on the left
-
Click "partner with person already on tree"
-
Pick the member from the other side of the family
-
Click "Add as partner"
There will be "duplicates" in your tree. This just means that they person appears in the tree more than once, because they are related in more than one way. People often see the word "duplicate" and think that that is an indication that they need to delete a duplicate. That is not the case.