[SPARK-12235][SPARKR] Enhance mutate() to support replace existing columns.

Make the behavior of mutate more consistent with that in dplyr, besides support for replacing existing columns.
1. Throw error message when there are duplicated column names in the DataFrame being mutated.
2. when there are duplicated column names in specified columns by arguments, the last column of the same name takes effect.

Author: Sun Rui <rui.sun@intel.com>

Closes #10220 from sun-rui/SPARK-12235.