How to stop duplicates contacts entering Salesforce?

To stop duplicate contacts, accounts and leads from entering Salesforce do the following:

1. Create Duplicate Matching Rules: Setup > Data.com Administration >  Duplicate Management  > Matching Rules
2. Create Duplicate Detection Rules:  Setup > Data.com Administration >  Duplicate Management > Duplicate Rules.
3. To clean all the existing duplicates use dedup-manager, below are a few exmaples

Dedup-Manager in Action:

Dedupe sub tabs (eliminate existing duplicates)