Fix mysql to support duplicated column names

This will fix the [broken test](https://github.com/rails/rails/commit/4a2650836680f51490e999c3c8441a2f9adff96e)  `test_with_limiting_with_custom_select`.

The query's result was built in a hash with column name as key, if the
result have a duplicated column name the last value was
overriding the first one.