You import thousands of records into your Simply CRM account. It’s not possible to find out duplicates in your .csv file at a glance and you finally end up importing duplicates, too. That’s the reason why we suggest you to enable duplicate record handling in the step 3 of import process. This will help you segregate duplicates from the list of other records. Not only does it help you in tracing out duplicates, but it also gives you the ability to manage them according to your requirements.
For more details refer the following link
Also, you need to be “heads up” while mapping records in your .csv file with existing contacts and organizations. Here’s a list of quick steps to help you import records successfully without duplicates.