completed assignment

force-app/main/default/triggers/DuplicateContact.trigger

@@ -0,0 +1,25 @@
+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!');
+            }
+        }
+    }
+}