Fix `brew update` so that it doesn't show doubles

Under certain conditions, `brew update` will show doubles. It's a rare
edge-case, but if you have a tap where the name of the formula is identical
with the name of the insalled file, then you see output like this:

    ==> Updated Formulae
    aalib				   ssh-copy-id
    casperjs				   sshuttle
    jack				   telemachus/anytap/brew-any-tap
    mogenerator				   telemachus/anytap/brew-any-tap

I believe this should fix that.

Closes Homebrew/homebrew#18495.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/update.rb

@@ -182,7 +182,7 @@ class Report < Hash
     formula = select_formula(key)
     unless formula.empty?
       ohai title
-      puts_columns formula
+      puts_columns formula.uniq
     end
   end