You can merge contacts automatically using Custom Merge Rules

Steps

Go to the Contacts app - Access the Contacts App (Manage Contacts page)
Press the More Options button
Select Merge Contacts
You arrive on the Merge Contacts page
Press the Merge Contacts button
Select Merge contacts automatically
You arrive on the Custom Merge Rules page
Press Create a Merge Rule
1. Give this Custom Merge Rule a name
2. Decide how you want your Custom Merge Rule to work by choosing the field(s) that need to be identical in order for eligible contacts to be merged
3. Press save
You arrive back on the Custom Merge Rules page. From here you can
1. Edit a Custom Merge Rule
2. Deactivate or Reactivate a Custom Merge Rule
Whenever there are contacts who match your rule's criteria, they'll be merged automatically. This happens within 2 hours of the duplicate having been created.

Makerble User Guide