Recently I've stumbled upon a problem of double line breaks while saving content using the CKEditor module.
Cause of Double Line Breaks
There's a setting in admin/config/content/formats/full_html (or the other relevant text type that you use, where the double line breaks appear), that causes the double line breaks to appear when you save content, even if text format is not 'paragraph'.
How to Solve This?
Go to 'admin/config/content/formats/full_html' and uncheck '' and this should solve the problem.