Main is checked twice

[zakk4223]

Main_Mister is added to CORE_URLS twice due to being pulled in via the Arcade core list in the wiki. It comes from the MRA format description link at the bottom.

Doesn't really cause an issue other than hitting the URL again. I only found it due to some local modification I made to pull main MiSTer binary from an alternate repo.

Although I guess it does expose the fact that any links in that wiki list that just happen to point to something with 'MiSTer' in the name will get caught up into CORE_URLS