fix: for issue when there are multiple aggregates with the same data field and aggregation operation

[JessamineQ]

PR Description

Fixes #6184.
When there are multiple elements in AggregateTransform with the same data field and aggregation operation, the code is currently only keeping the first output field name.

Checklist

• This PR is atomic (i.e., it fixes one issue at a time).
• The title is a concise semantic commit message (e.g. "fix: correctly handle undefined properties").
• yarn test runs successfully
• For new features:
  • Has unit tests.
  • Has documentation under site/docs/ + examples.

[domoritz]

Thank you for the pull request and your patience!