[Users] Adding an address adds another address as well

[John Jason Jordan]

Claws-Mail 3.16.0 on Xubuntu 18.04, up to date.

This is the weirdest thing ever. I have an address list with 20-30
addresses that I mail to now and then - including this address for
Claws-Mail listserv.

Suddenly, when I have the compose window open and I select the address
for the LibreOffice list it adds another address as well. This only
happens with the LO address. The additional address is hidden, above
the normal line in the compose window, but I can see it if I use the
tiny scroll bar to the right of the address line.

The address that is added is always the same address, like it's a
default or something. But it's not; just now composing this message it
did not get added.

It must have something to do with that one address, but I've looked
everywhere I can think of and I haven't found anything. I want to know
how this happened, so I can make sure it never happens again. Or if
it's a bug, how do I report it?