Any way to only add a header when there's not one there already?

On Thu, Mar 21, 2013 at 03:39:43PM +0000, Chris Green wrote:
> ... and I have finally found what the interface from gmane to DBA needs,
> a Content-Type: header.  I have added this to the bottom of my
> attributes file:-
> 
>     x_headers=Content-Type: text/plain; charset=UTF-8; format=flowed
> 
> and now it seems to work OK.
> 
However, it only works for posts I initiate.

If I followup a post there will already be a Content-Type: header there
and my posting gets rejected because of the duplicate Content-Type:

Is there any way to get tin to only add a header if there's not one of
the same type there already?  Or could I somehow force tin to add a
Content-Type: header by making my text non-ascii, then it would
presumably do it right in that it wouldn't duplicate the header.

-- 
Chris Green