Demonstrate the Set-like nature of Options collections

Options collections are backed by Sets, and thus trying to push a new
option with a name that duplicates an existing option cannot succeed.

Later, we can exploit this behavior and remove some conditionals.

Library/Homebrew/test/test_options.rb

@@ -54,6 +54,15 @@ class OptionsTests < Test::Unit::TestCase
     assert_equal 1, @options.count
   end
 
+  def test_preserves_existing_member_when_pushing_duplicate
+    a = Option.new("foo", "bar")
+    b = Option.new("foo", "qux")
+    @options << a << b
+    assert_equal 1, @options.count
+    assert_same a, @options.first
+    assert_equal a.description, @options.first.description
+  end
+
   def test_include
     @options << Option.new("foo")
     assert @options.include? "--foo"