As far as I've ever seen the problem is cause by invalid HTML.
Paste in valid, correctly formatted HTML and it doesnt happen.
I might be wrong, but that's what I think is happening.
I believe what is happening is you're pasting in HTML where the closing HTML tags are missing. The HTML editor we use on our site then attempts to it's best to correct those errors and the results is the extra lines. One for each incorrectly formated paragraph HTML tag paged in via Turbo Lister or other application.
I verified this in a post once before and even showed the correct version of the HTML and verified that if that correct version is pasted in that this doesnt happen. I think that was about 2 months ago now.
Maybe you can try pasting from Turbo into some other program for HTML editing. Then save it there. Then copy the HTML from that html editor program into our site. By doing that you'd be copying and pasting from a known application and not a fuddy duddy HTML editor built into Turbo Lister.