A matching rule is a rule that defines how records are compared for duplication based on certain fields or fuzzy logic. By including more fields or changing the matching method, you can make the matching rule more or less strict and reduce false positives or negatives. In this case, including the email field to the existing matching rule for contacts can help avoid duplicate messages for common names by checking if the email addresses are also identical. References: https://help.salesforce.com/s/articleView?id=sf.
matching_rules_create.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.
matching_rules_considerations.htm&type=5