Kernel.#local_variables should not contain duplicated names

This patch fixes following behavior.

----


Example
```shell
ruby -e 'foo = true; p local_variables'
```

result - MRI(ruby 2.1.0dev (2013-12-03 trunk 43982) [x86_64-linux])
```
[:foo]
```

result - rubinius(master at 7820d0d3445b68dc36bbbbf15c61fdc8a5df976a)
```
[:foo, "foo"]
```