Return only unique source documents

[valeksiev]

Describe the background of your pull request

With the chunking in place on ingestion, multiple chunks from a single document can be used to answer a question. This would result in redundant source links in the response. This PR fixes that.

Additional context

Add any other context

Governance

• Documentation is added
• Test cases are added or updated

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• I've read the contributing document https://github.com/alkem-io/.github/blob/master/CONTRIBUTING.md.
• I've read and understand the Code of Conduct https://github.com/alkem-io/.github/blob/master/CODE_OF_CONDUCT.md.
• I understand that any contributions or suggestions I made may make it into the actual code. I've read the License
  https://github.com/alkem-io/Coordination/blob/master/LICENSE.
• I understand that submitting the pull request requires agreeing to and signing the contributor license agreement.