This is often down to the HTML that the program doing the copy generates. This is far from a trivial issue.
I'll need to look at this on a Mac, but on Windows, it actually generates an extra break. This is because it generates <p> tags but puts a 0cm margin-bottom of them (which isn't dissimilar from the way our own editor works). However, we treat <p>s as requiring a double break, which ends up inserting an extra one.