orclx> conn system/oracle
Connected.
orclx> drop user jw;
drop user jw
          *
ERROR at line 1:
ORA-01918: user 'JW' does not exist


orclx> grant create session,create any context to jw identified by jw;

Grant succeeded.

orclx> create context system_context using system_package;

Context created.

orclx> conn jw/jw
Connected.
orclx> create or replace context system_context using jw_package;

Context created.

orclx>

SQL> create or replace trigger trg_pre_ddl before ddl on database
  2  declare
  3    dummy pls_integer;
  4  begin
  5    if ora_sysevent = 'CREATE' and ora_dict_obj_type = 'CONTEXT' then
  6      begin
  7        select 1 into dummy from dba_context where namespace = ora_dict_obj_name;
  8        raise_application_error (-20000, 'Context already exist, you can''t overwrite it');
  9      exception when no_data_found then null;
 10      end;
 11    end if;
 12  end;
 13  /

Trigger created.

SQL> drop context test_ctx;

Context dropped.

SQL> create or replace context test_ctx using pkg;

Context created.

SQL> /
create or replace context test_ctx using pkg
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-20000: Context already exist, you can't overwrite it
ORA-06512: at line 7