Hi @mohini1329,
as the others have already suggested, you should make sure that each row in the Pandas DataFrame has a unique index.
Dropping/resetting the index is not the best option though, because KNIME uses the RowID (which is equal to the DataFrame index) to identify rows when you select / highlight individual cells in your data. If you reset the index, then this row identification will no longer work.
The best solution would be to let the index of all values in your input table as they are, but assign new unique indices (which are actually string based) to all new rows that you add.
@mlauber71: the RowID node that you mentioned has the same problem if “enable hiliting” is disabled.
If you do not care about selection/hiliting, then dropping the index is fine, though 
Hope that helps,
Carsten