[Mailman-Users] repeated subject prefix

[N K]

I am having a problem with the prefix added before the
subject. The prefix is set at [listname] in the
general options page, and emails sent to the list have
subjects of the form: 

[listname] Re: [listname] Re: [listname] Re:
(...etc...) Re: [listname] Original_Subject

As I haven't noticed any other lists have this
problem, there seems like there must be some
fundamental setting that I am missing, but I can't
find it. Can anyone tell me if/how I can set it to be
just

[listname] Re: Original_Subject

or something short like that?
Thanks,
Nathan

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