Hi,
Sometimes You can get “TNS-01106: Listener using listener name string has already been started ” error.
Details of error are as follows.
TNS-01106: Listener using listener name string has already been started
Cause: Another listener is already listening on one of the ADDRESSes specified.
Action: Shutdown the previous listener specified by the listener name before starting this one.
This error is related with Two or more listeners have same port, and listeners cannot have same port on same server.
Check your listeners and change the listeners port and they should be different. If one listener port is 1521, then another one should be 1522 or 1523.
Do you want to learn more details about RMAN, then read the following articles.
https://ittutorial.org/rman-backup-restore-and-recovery-tutorials-for-beginners-in-the-oracle-database/