I invited/created a new user in the App but by mistake did this twice.
So now the same username shows up twice in the Nuki Android App and Web management!?
Wondering which one to delete in order to maintain the user which currently has access to my Nuki Opener and Smartlock v2!?
In the Nuki App in user management tap on the user and then on the 
icon to get information on create date and last action date of a user. This should help you to find the correct one to keep/delete.
If you did the invites from the Nuki Appbut (“Added via Nuki App”) want to handle those invites for Smart Lock and Opener as a single user in Nuki Web, you can create a (new) user with E-Mail within Nuki Web (“Added via Nuki Web”) and merge the App invites to it.
Thanks, got it!
Wondering if this kind of user management and multiple instances can be enhanced a bit as it could be confusing to manage all users with ease.
The main cause for this multi-user split is, that all authorizations are created and stored at the Smart Lock / Opener itself. So - as long as you do not start the process from Nuki Web - Nuki Web has no chance to know that authentications from different devices (or also the same) belong to the same user if you do not merge them manually.
If you really need this advanced user management (especially across devices) it is recommended to do all invites from Nuki Web via e-mail to keep the user-structure consistent.