Salesforce has released an update to the Duplicate matching that has broken our custom Find Duplicate matching process on Lightning.
Salesforce now has active duplicate detection. Every time that you open a Contact record it searches for duplicates and alerts you.
Please amend the duplicate detection rules in your ORG to search phone numbers and email addresses and whatever other field you think detection makes sense on (e.g. first name and last name - but be careful as there could be more than one "John Smith" in your database).
The duplicate and matching rules can be found via Setup > search Matching rule or Duplicate in Quickfind Box.
1. Add your matching rule:
2. Activate the rule after saving.
3. In "Duplicate Rules" edit "Standard Contact Duplicate Rule" to add your matching rule:
4. Then add the Lightning widget "Potential Duplicates" to the Lightning page layout where you want it.
5. Please remove the Propertybase Find Duplicates button (SF1 and Lightning Action button) from the contact object's page layouts.
After setting up Duplicate Finder, if it finds a match it will tell you, when you open the contact record. There is a View Duplicate link that enables to you merge the duplicate(s).
Then when you click View Duplicates, you can move forward as normal.
More setup and usage details on this feature can be found in Salesforce resources:
Overview article: https://help.salesforce.com/articleView?id=duplicates_existing_setup.htm&type=0