Looking at the code, it appears the plugin will not automatically associate your token's id with an existing user's uid when they overlap (likely for security reasons). It checks the database hash 'appId:uid' for the association. The hash fields are your app's id and the value is the associated uid. You should be able to manually create an association with an existing user by editing that hash.
I think the mongo command would be like this:
db.objects.update({ _key: "appId:uid" }, { $set: { "13": "13" } })
