private static boolean add(RowResolver rrToAddTo, RowResolver rrToAddFrom,
      IntRef outputColPosRef) throws SemanticException {
    return add(rrToAddTo, rrToAddFrom, outputColPosRef, -1);
  }
Adds column to RR, checking for duplicate columns. Needed because CBO cannot handle the Hive
behavior of blindly overwriting old mapping in RR and still somehow working after that.
@return True if mapping was added without duplicates.
ql/src/java/org/apache/hadoop/hive/ql/parse/RowResolver.java