fix UI

src/main/ipc-handlers.ts

@@ -106,11 +106,26 @@ function registerGetDomains(): void {
 
 /**
  * Create a new domain
+ * Deduplication: Check if domain+username already exists
  */
 function registerCreateDomain(): void {
   handleWithParams<CreateDomainParams, Domain>(
     "createDomain",
     async (params) => {
+      // Check for duplicate domain+username
+      const existingDomains = await listDomains();
+      const duplicate = existingDomains.find(
+        (d) =>
+          d.domain.toLowerCase() === params.domain.toLowerCase() &&
+          d.username.toLowerCase() === params.username.toLowerCase()
+      );
+
+      if (duplicate) {
+        throw new Error(
+          `Domain "${params.domain}" with user "${params.username}" already exists. Please edit the existing domain instead.`
+        );
+      }
+
       const now = new Date().toISOString();
       const domain: Domain = {
         id: uuidv4(),