pandas.errors.MergeError: Merge keys are not unique in either left or right dataset; not a one-to-one merge

This error occurs when the merge keys (the columns used to merge two dataframes) have duplicate values in either the left or right dataframe.

For example, if you have two dataframes A and B that you want to merge on a column called "ID", and the ID column in A has duplicate values (e.g. two rows have ID=1), you will get this error when trying to merge A and B.

To resolve this issue, you can either drop the duplicate values in the merge key column using the `drop_duplicates()` method, or choose a different column to use as the merge key.