Salesforce announced a new feature in Summer '16 to Relate a Contact to Multiple Accounts.
This is a latest feature to associate single contact to multiple accounts in Salesforce to reduce the duplicate contact data.
To enable this,please follow below path on Salesforce setup
Go to Setup => Customize => Accounts =>Account Settings and check the checkbox (read the warning carefully) and hit the save button.
After save successfully your screen will look like below screen
Add "Related Contacts" related list on Account Page Layout
Add "Related Accounts" related list on Contact Page Layout