[Exim] question on duplicate headers

While doing some testing yesterday on a filter, I noticed some potentially
interesting behaviour. The filter I was working on was to catch mail that
has a specific header that is potentially added to all of our incoming
mail. To test it, I bounced myself a message which already had the
header. To my surprise, the (previously working) filter rule didn't fire.
After poking around a bit I found that when I "put the ball back into
play", the header in question was added to the message again, and that
exim... I'm not sure what exim did, to be honest, but it didn't populate
the appropriate $header_ variable, and so the filter rule failed. When I
deleted the duplicate header and delivered it the rule fired and
everything worked.

I guess my question here is, is this expected behaviour? I'm sure at some
point one of our users will bounce one of these messages to another, and
the rule won't fire, and hyjinks will ensue. I'm open to the possibility
that maybe the other piece of software is screwing up because it shouldn't
be adding duplicate headers, I'd just like some thoughts, if available =).

Thanks
--john jetmore