@Kushagra Sharma Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
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Firstly the index name added for the data source will be used to reference this data source. A new cognitive search index with the provided name will be generated after data ingestion is complete
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Please note that the behavior you are seeing with respect to the indexing the data source is expected.
As you can see in the below screenshot, where I tried adding an index abcd-index for my Cognitive Search resource:
Within the Cognitive Search resource, there are 2 indexes being created:
However after the processing is completed, the abcd-index-index will be removed (deleted automatically) and only abcd-index will remain. See below:
Hope this helps. If you have any follow-up questions, please let me know. I would be happy to help.