This is a well known problem with DOS derived systems (basically DOS and Windows). Within a file there needs to be a way to mark the end of a record, the two main methods are to either know the length of a line or else to put in a marker. Historically this has been "line feed" (LF, ASCII 10d), also often seen as "\n". C language files use the ASCII NULL (value 0) code.
Early in DOS's design it was assumed that all text files would be printed, and so were stored in printer format. Each line ended with a carriage return (go to the left of the paper) and line feed (go up a line), commonly called CRLF. This has persisted, and DOS/Win text files all have CRLF at the line end, whereas other systems just have LF. Some software can be fooled by this: either the file runs on, just going down the page but never returning, or else you get double spacing.
DOS file under most other OSs:
Line 1
Line 2
Line 3
Other (today typically Linux) under DOS:
Line 1
Line 2
Line 3
Under DOS/Win Wordpad seems to be more aware than Notepad. Other than that, edit the file in the browser and remove the extra lines by hand. :-)