I'm resurrecting this thread with a solution. After having the same thing happen to me, I did some reading.
Basically it happens when you cut and paste from a html or webpage or from MS Word into a mail message. It arises from a mixup between html code / MS Word text formatting code and Apple's rich text message code.
The end result is that anything you type after the cut and paste is also double spaced.
Workarounds:
- change message format to plaintext (can't remember if this worked)
or
- paste into notepad / text edit and change to plain text then repaste into mail message
or
- type a bit more, or press return a couple of times, then paste into the middle of the text block, and continue typing from the end of the previously entered text, not from the end of the material you pasted. This means you are continuing with the original single-spaced format. Any editing of the pasted block will still come out double-spaced though.