trigger DuplicateContact on Contact (before insert) {
    Set<String> emailAddresses = new Set<String>();

    for (Contact contact : Trigger.New) {
        if (emailAddresses.contains(contact.Email)) {
            contact.Email.addError('Email already exists!');
        } else {
            emailAddresses.add(contact.Email);
        }
    }

    List<Contact> existingContacts = [
        SELECT Id, Email 
        FROM Contact 
        WHERE Email IN :emailAddresses
    ];

    for (Contact contact : Trigger.New) {
        for (Contact existingContact : existingContacts) {
            if (contact.Email == existingContact.Email && contact.Id != existingContact.Id) {
                contact.Email.addError('Email already exists!');
            }
        }
    }
}