I'll try a str_replace and report back.

Adding this in the appropriate location seems to have helped.

$notify_message = str_replace("\r", '', $notify_message);

I had done this in my own code, yet it wasn't working. But again, that's because I was uploading my own code to the new server, yet messages were still being created on the old server.

Time to re-extract these methods into a plugin for my own use.

What are the strings in the code that generate the email? If the emails are plaintext I'd guess unix line endings will give you one line, whilst windows line endings (crlf) would give you two lines?

They're \r\n. I had changed them earlier and it seemed to have little effect, but come to think of it, that may have been because I was only uploading the changes to my new server, and some folks were still hitting the old one. Except… I never had any problems with this double-spacing on the old server.

At any rate, replacing \r\n with just \n on the new server fixes things except in the comment itself. There, it's still doubled. I'll try a str_replace and report back.