Modifying a message within procmailrc

I am experimenting with a program that adds headers to a message.

I was told to use

:0
* ! already-processed-flag
| program | $SENDMAIL -oi myname(_at_)mysite(_dot_)com

I verified from the command line 

   program < message > message.modified
   diff message message.modified

that if stdin is a "normal" message, then stdout is the message with the
extra header added and no other changes.  So I wonder whether a better
approach would be

:0 fhw
| program | $SENDMAIL -oi myname(_at_)mysite(_dot_)com

Actually, the recommended approach does not seem to work.  I added the 'c'
flag because I was suspicious.  Result: I get two log files for each
message, but the only copy I ever see is the unmodified one.