When opening an image, Pillow loops through the different possible formats.

If I add a print statement to see which formats it checks,

    def _open_core(fp, filename, prefix, formats):
        for i in formats:
            print(i)
            i = i.upper()

and then run

from PIL import Image
Image.open("Tests/images/hopper.spider")

I get

BMP
DIB
GIF
JPEG
PPM
PNG
BMP
DIB
GIF
JPEG
PPM
PNG
BLP
BUFR
CUR
PCX
DCX
...

The first set of formats are checked twice - BMP, DIB, GIF, JPEG, PPM and PNG.

This is because after the preinit formats fail to load the SPIDER image, Pillow initialises the rest of the formats, and then just checks all the formats again, even the preinit ones that already failed.

So this PR excludes the already checked formats the second time.