fix: prevent duplicate contacts during QR code scanning

- Add explicit duplicate contact check before database insertion
- Show warning notification when duplicate contact is detected
- Return early to prevent duplicate database entries
- Improve user feedback for duplicate contact scenarios

This change ensures users get clear feedback when scanning an already-added contact and prevents duplicate entries in the contacts database.

src/views/ContactQRScanShowView.vue

@@ -574,6 +574,26 @@ export default class ContactQRScanShow extends Vue {
     try {
       logger.info("Opening database connection for new contact");
       await db.open();
+      
+      // Check if contact already exists
+      const existingContacts = await db.contacts.toArray();
+      const existingContact = existingContacts.find(c => c.did === contact.did);
+      
+      if (existingContact) {
+        logger.info("Contact already exists", { did: contact.did });
+        this.$notify(
+          {
+            group: "alert",
+            type: "warning",
+            title: "Contact Exists",
+            text: "This contact has already been added to your list.",
+          },
+          3000,
+        );
+        return;
+      }
+
+      // Add new contact
       await db.contacts.add(contact);
 
       if (this.activeDid) {