## What changes were proposed in this pull request?
After https://github.com/apache/spark/pull/12067, we now use expressions to do the aggregation in `TypedAggregateExpression`. To implement buffer merge, we produce a new buffer deserializer expression by replacing `AttributeReference` with right-side buffer attribute, like other `DeclarativeAggregate`s do, and finally combine the left and right buffer deserializer with `Invoke`.
However, after https://github.com/apache/spark/pull/12338, we will add loop variable to class members when codegen `MapObjects`. If the `Aggregator` buffer type is `Seq`, which is implemented by `MapObjects` expression, we will add the same loop variable to class members twice(by left and right buffer deserializer), which cause the `ClassFormatError`.
This PR fixes this issue by calling `distinct` before declare the class menbers.
## How was this patch tested?
new regression test in `DatasetAggregatorSuite`
Author: Wenchen Fan <wenchen@databricks.com>
Closes#12468 from cloud-fan/bug.
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