Often times I cut / paste between systems and end up with duplicate lines in Notepad++. Googling finds a lot of solutions that don't work. Here's all you need to do-
Hit <Ctrl> <H> to get to the replace box (or go to the Search menu and click Replace.)
In "Find What", put \r\n\r\n
In "Replace", put \r\n
Under Search Mode, check "Extended".
Click "Replace All".