Add spec aliasing the same global variables twice

Author: eregon

spec/ruby/language/alias_spec.rb

@@ -255,4 +255,9 @@ def test_with_check(*args)
     code = '$a = 1; $b = 2; alias $b $a; p [$a, $b]; $b = 3; p [$a, $b]'
     ruby_exe(code).should == "[1, 1]\n[3, 3]\n"
   end
+
+  it "supports aliasing twice the same global variables" do
+    code = '$a = 1; alias $b $a; alias $b $a; p [$a, $b]'
+    ruby_exe(code).should == "[1, 1]\n"
+  end
 end

spec/tags/language/alias_tags.txt

@@ -1,3 +1,4 @@
 slow:The alias keyword on top level defines the alias on Object
 slow:The alias keyword can create a new global variable, synonym of the original
 slow:The alias keyword can override an existing global variable and make them synonyms
+slow:The alias keyword supports aliasing twice the same global variables