Write a trigger to prevent duplicate Contacts based on Email and Phone when inserting or updating a Contact.

trigger ContactTrigger on Contact (before insert, before update) {
    Set<String> emails = new Set<String>(), phones = new Set<String>();
    for (Contact c : Trigger.new) {
        if (c.Email != null) emails.add(c.Email);
        if (c.Phone != null) phones.add(c.Phone);
    }
    Set<String> seen = new Set<String>();
    for (Contact e : [SELECT Id, Email, Phone FROM Contact
                      WHERE (Email IN :emails OR Phone IN :phones) AND Id NOT IN :Trigger.new]) {
        seen.add(e.Email + '|' + e.Phone);
    }
    for (Contact c : Trigger.new) {
        String key = c.Email + '|' + c.Phone;
        if (!seen.add(key)) c.addError('Duplicate Contact with same Email and Phone.');
    }
}