I have had the same problem with DW - but it only happens on my home computer, not my office computer. Go figure. I have thoroughly searched through the dw help info and found nothing but I'm sure it's a setting somewhere.
It's really frustrating because I go back and forth a lot, and by the time I'm done with a file after opening it several times and saving it at home, it's triple-spaced or quadruple-spaced even.
I know, this is NO help to you, but at least I can commisserate 🙂
-Elizabeth