> Were the two labels actually applied to the same email or to different
> emails in the same conversation?
I thought of that too. Nope. It was one message in that
conversation. (But there is a bit more to this ... see later.)
> It's either that, or I know I can force it with IMAP, are you using IMAP at
> all?
No, not using IMAP. Webmail only.
Here is how I think it happened. I apparently received two identical
Spam messages, sent via different email lists. For some odd reason,
Gmail apparently marked one of them as spam but not the other. Gmail
then recognized them as duplicates, so it dutifully trashed the second
one (as in erased, gone, no chance of recovery) ... leaving me with
*one* message sitting in my Inbox with both Spam and Inbox labels
attached to it.
That's the best I can make of it.
I am guessing that when Gmail auto-deletes a duplicate email for you,
their algorithm must transfer the labels from the deleted message to
the other message that remains. That seems reasonable.
Andy