extend/os/mac/keg_relocate: fix relocation of duplicate `RPATH`s

Homebrew/homebrew-core#77623 revealed two bugs. One in `ruby-macho`,
which turns out to be unable to delete duplicated `RPATH`s. This was
fixed with `ruby-macho` 2.5.1.

The second, which this commit fixes, is in our handling of duplicate
`RPATH`s. Since we iterate over each `RPATH`, attempting to relocate the
first duplicate fails since it will no longer exist after having
relocated the original.

Library/Homebrew/extend/os/mac/keg_relocate.rb

@@ -94,6 +94,7 @@ class Keg
   def each_linkage_for(file, linkage_type, &block)
     links = file.method(linkage_type)
                 .call
+                .uniq
                 .reject { |fn| fn =~ /^@(loader_|executable_|r)path/ }
     links.each(&block)
   end