I've experienced this issue since I had my laptop swapped. I copied my old signature from a previous email, and then pasted it, saved it and made it my default. I then had the weird issue with it inserting 2 blank lines before my signature instead of one.
Done a bit of Googling today, found that I'm not the only person with the issue, but the answers being offered didn't help me.
Decided to test on a new email signature, and that worked fine, not double lines. So I copied my email signature into Notepad to remove any formatting, then pasted it back in. For any part of my sig with hyperlinks i added the links in via Word, then pasted back into my sig, and it works.
Essentially, there must be some weird hidden formatting which has happened whilst copying my sig over. Fix is to completely start over, and either re-type it in, or copy into Notepad to remove the formatting, paste back in and then reformat in Outlook.
Hope that helps!