When dealing with lots of data duplicates are almost always inevitable.
If you believe you have duplicates in your data then it is best to manage them in your spreadsheet before importing. Read this article on how to remove duplicates in Microsoft Excel or Google Sheets.
Your system won't allow for two individuals with the same email to be imported so one of them will be rejected from the import.
If you do happen to have individuals imported with the same name, these will show up in the 'Duplicates' group in the system, and you can merge them from there by selecting the two individuals.