"It looks like you are creating a duplicate Account/Contact/Lead. We recommend you use an existing Account/Contact/Lead instead."

This validation error is caused by the duplicate management rule. In order to import such records, you should follow the guide below to deactivate the rule:

1. Go to Setup > Administer

2. Select Data.com Administration > Duplicate Management > Duplicate Rules

3. Select the rule(s) to deactivate