Split from this thread.
That's what happens to me also (using Outlook 2016 in Office 365), except that the extra spacing doesn't appear until after the email has been sent. So everything looks normal after typing the email, but the sent version is double spaced.
It doesn't happen on the original send; it happens when replying to an email from someone else.
It doesn't happen with all correspondence; the recipient's email program is also implicated.
Certainly, however, it happens when I'm corresponding with another Outlook user.