Ignore duplicate rpaths in #change_rpath

This causes a deviation in behaviour from `install_name_tool`, which
will happily change an existing rpath to one that already exists.

Closes #436.

Author: carlocab

lib/macho/macho_file.rb

@@ -381,11 +381,9 @@ def rpaths
     #  rpaths simultaneously.
     # @return [void]
     # @raise [RpathUnknownError] if no such old runtime path exists
-    # @raise [RpathExistsError] if the new runtime path already exists
     def change_rpath(old_path, new_path, options = {})
       old_lc = command(:LC_RPATH).find { |r| r.path.to_s == old_path }
       raise RpathUnknownError, old_path if old_lc.nil?
-      raise RpathExistsError, new_path if rpaths.include?(new_path)
 
       new_lc = LoadCommands::LoadCommand.create(:LC_RPATH, new_path)
 

test/test_macho.rb

@@ -574,10 +574,6 @@ def test_rpath_exceptions
       file.change_rpath("/this/rpath/doesn't/exist", "/lib")
     end
 
-    assert_raises MachO::RpathExistsError do
-      file.change_rpath(file.rpaths.first, file.rpaths.first)
-    end
-
     assert_raises MachO::RpathExistsError do
       file.add_rpath(file.rpaths.first)
     end