ORA-20500 ORA-06512 ORA-06512 (Error while adding target into OEM)

ERROR:

testab1 (oracle_database)

ORA-20500: Target Already exists: testab1:oracle_database:oracle.linux.net:https://oracle.linux.net:3872/emd/main/:3:0

ORA-06512: at "SYSMAN.EM_TARGET", line 3998

ORA-06512: at line 1

SOLUTION:

Log in to the EM console.

Select Setup > Add targets > Auto Discovery Results.

Select the database (from the Ignored Targets tab)

Now, try to add database and at the last step while saving, you won’t be getting an error.

Reference: 

Enterprise Manager Cloud Control 12c - Manually adding a database target fails with ORA-20500 (Doc ID 1523394.1)