[SPARK-17866][SPARK-17867][SQL] Fix Dataset.dropduplicates

## What changes were proposed in this pull request?

Two issues regarding Dataset.dropduplicates:

1. Dataset.dropDuplicates should consider the columns with same column name

    We find and get the first resolved attribute from output with the given column name in `Dataset.dropDuplicates`. When we have the more than one columns with the same name. Other columns are put into aggregation columns, instead of grouping columns.

2. Dataset.dropDuplicates should not change the output of child plan

    We create new `Alias` with new exprId in `Dataset.dropDuplicates` now. However it causes problem when we want to select the columns as follows:

        val ds = Seq(("a", 1), ("a", 2), ("b", 1), ("a", 1)).toDS()
        // ds("_2") will cause analysis exception
        ds.dropDuplicates("_1").select(ds("_1").as[String], ds("_2").as[Int])

Because the two issues are both related to `Dataset.dropduplicates` and the code changes are not big, so submitting them together as one PR.

## How was this patch tested?

Jenkins tests.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #15427 from viirya/fix-dropduplicates.