double sound from T-bird

[Eben King]

Hi, I have Thunderbird 128.11 on Debian 12.11 (Bookworm).  Thunderbird 
works fine (well, except for trailing spaces) but when I get mail it 
plays the new mail sound twice, a few ms apart.  Sometimes it sounds 
like a flanger effect, sometimes it sound like the kind of echo you 
might get in a hallway.  Either way, it probably shouldn't happen.  Any 
guesses as to why it might be?  The only reasonable guess I can come up 
with is that it plays the sound once when the mail arrives, and again 
once it's been filtered and lands in a different folder.