To connect to a Windows machine as a user who is already logged in, do NOT enter any user credentials (Domain, User or Password) in the Connection Properties window.
Windows connections can only have one set of credentials in use at a time. If a user connects to a machine by using NO user name and password, and then disconnects and reconnects with an Administrator password, Windows returns an ERROR_SESSION_CREDENTIAL_CONFLICT error.
For example, if you have mapped a drive to the machine \\serverA\share while logged in as serverA\administrator, you CANNOT map another drive to the same machine while logged in as serverA\user.
You can, however, connect to the same machine with different credentials if you map to the machine via its IP address.
In the example above, if serverA has the IP address 192.168.1.100, you CAN employ the net use command to connect to the machine \\192.168.1.100\share as serverA\user.