When I do a schema compare that has added a virtual column the alter table command that it generates has a bug in it.
The statement it generates is:
ALTER TABLE vendorportaluser.vendorcontacts ADD (dbupperusername VARCHAR2(30 BYTE) GENERATED ALWAYS AS (UPPER("DBUSER_NAME")));

Which results in error:
ORA-54015: Duplicate column expression was specified

The correct syntax is:
alter table VENDORPORTALUSER.VENDORCONTACTS add dbupperusername as (UPPER(DBUSER_NAME));