Fix top-level formula having duplicate deps

If the top-level formula being installed has any keg-only
dependencies, the lib/include/bin/pkg-config paths for those
deps will be added twice to the ENV vars.

This doesn't break anything, but does make debug output somewhat longer
than it needs to be.

Tidy this up by calling uniq on deps before iterating.

(The cause of this duplication is related to how top-level formula are
run, to support keeping patches in __END__ blocks.)

Fixes Homebrew/homebrew#1110

Library/Homebrew/install.rb

@@ -54,7 +54,7 @@ end
 def install f
   show_summary_heading = false
 
-  f.deps.each do |dep|
+  f.deps.uniq.each do |dep|
     dep = Formula.factory dep
     if dep.keg_only?
       ENV.prepend 'LDFLAGS', "-L#{dep.lib}"