This is actually a feature called Tab Grouping and it can be enabled horizontally or vertically. As far as I can tell, it's the same thing as a window split to enable you to see 2 or more sections of the same file simultaneously. The difference is that
Tab Grouping is saved between sessions i.e. if you close and re-open Visual Studio, this setting will be retained!
To disable it:
In your particular case, you have 3 files opened in each group. This means you will have to repeat the procedure 3 times either starting on the left or right side and moving each file either to the left or right side until you only have one group open.
In case somebody wants to enable it (or the horizontal version):
Googling around I was able to find numerous people complaining about all sorts of bugs with splitting/grouping code windows while using ReSharper. You could try removing ReSharper, ensure windows are not split/grouped, restart VS to verify if it worked, then re-install ReSharper?