From this link: "The glyphs are a result of both erosion of the stone surface (evident elsewhere in the temple) and the process of filling in and re-carving the stone to replace some of the original hieroglyphics. The technical term for such a surface that has been written on more than once is a palimpsest. The usurping and modifying of inscriptions was common in ancient Egypt throughout its history. The Abydos glyph was modified at least once in antiquity, and perhaps twice. Some of the filling has fallen out in places where the older and the newer inscriptions overlap, and the result is unique and odd-looking.
The text is part of the titulary of Ramesses II and can be translated as "The one of the Two Ladies, who suppresses the nine foreign countries." This replaces the royal titulary of Seti I that was originally carved into the stone. More technically, the actual "helicopter" seems to be a portion of the psd.t sign and the X3s.t sign on top of each other, with portions missing. An apparent change in scale also mucks things up."
Another site about this
And another one