declare
 v_busy_xyz_processes number;
begin
 for i in 1..100 loop
   -- is a copy of this code running already?
   select count(*) into v_busy_xyz_processes 
   from v$session where user_info = 'XYZ process';

   if v_busy_email_processes = 0
   then
     -- No running copy of this code found in v$session, so put 
     -- signature on this session to signal other processes
     -- that we are busy with XYZ.
     -- Now this signature will be visible in v$session.
     dbms_application_info.set_client_info( 'XYZ process' ); 
    
     --==================
     -- run XYZ here
     --==================
     ...

     -- remove signature - or if this code only runs
     -- in dbms_job, then it'll end naturally
     dbms_application_info.set_client_info( 'done' ); 

     exit;  <-- XYZ done, so exit loop
   else
     -- Sleep 5 sec, then try again. Hopefully the blocking
     -- session will be done.
     dbms_lock.sleep(5); 
   end if;
 end loop;
end;
/