Fix duplicate PPD->IPP media-type name mappings

Fix duplicate PPD->IPP media-type name mappings

If more than one MediaType in a PPD matched the pattern for a registered
PWG media-type name, they would all be mapped to that name, making it
impossible for a client using CUPS' IPP interface to distinguish between
them or specify a particular one.

This comes up with Epson-provided PPDs, which frequently hit this case
by combining print quality with media type like so:
*MediaType PLAIN_HIGH/Plain paper-High: "
*MediaType PLAIN_NORMAL/Plain paper-Standard: "

Solve this problem by not assigning the standard PWG name to either one
when such duplication occurs.