The Importr module allows to import data from any existing dataset including data from Salesforce external data sources. It prevents duplicates from being created in Salesforce as a result of import and at the same time, it allows you to update your existing records when new or more up to date information is found in another data source. 

Start by going to Importr module and hit Upload Data button, select From Dataset

Create dataset dialog will appear. Specify dataset to import form and a dataset to import to, click Save.

Dataset will be created and records will be grouped into two sections - Unmatched records and matched records.

Records can be reviewed, imported and enriched manually or en mass using Mass Import and Mass Update function.

Learn more:
Working with Imported CSV Records