The handle to \Device\Mup is to ensure the mapping re-use the same credentials as used previously.
The SMB implementation of Microsoft allows you the connect to a remote host with a single credential, if you connect to the same resource with different credentials you'll receive an error 1219:
System error 1219 has occurred. Multiple connections to a server or
shared resource by the same user, using more than one user name, are
not allowed. Disconnect all previous connections to the server or
shared resource and try again.
Actually this does applies per remote host but per hostname. So you can connect with different credentials if you connect via IP and hostname, each connection will appear as a new handle.