I am not able to create “one to many join” in Power BI. It shows an issue with my dimension table, tell me how can I identify that have a duplicate in my dimension/one-side table.

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Way to check duplicates in the table?

To identify duplicates in a table, create a table visual using the key or join column. Set the column to “Don’t Summarize” and display it as a Count. Apply a visual-level filter with a count greater than 1. If any rows appear, it indicates that duplicates are present.

You can remove duplicates in the Power query.

Home Tab -> Remove Rows -> Duplicate Rows