To reproduce, after vim --clean:

:se splitbelow
:nnoremenu 1.10 WinBar.Next :bn<cr>
:split

I could not reproduce the duplicated status line with --clean, but in my setup, adding a window toolbar messes up things quite a bit. It's better seen through a screencast:

You may also see how repeatedly entering and exiting the CtrlP plugin raises the status line each time.