diff --git a/.gitignore b/.gitignore
index 8c70e19..4ab77ac 100644
--- a/.gitignore
+++ b/.gitignore
@@ -46,5 +46,11 @@ node_modules/
 npm-debug.log*
 yarn-error.log*
 
-.claude/
-.abacusai/
\ No newline at end of file
+.claude/*
+.abacusai/*
+graphify-out/*
+.graphify_ast_extract.py
+.graphify_ast.json
+.graphify_detect.json
+.graphify_python
+.graphify_uncached.txt
\ No newline at end of file
diff --git a/app/Controllers/CustomerController.php b/app/Controllers/CustomerController.php
index 37e82a4..b52d321 100644
--- a/app/Controllers/CustomerController.php
+++ b/app/Controllers/CustomerController.php
@@ -91,6 +91,21 @@ class CustomerController extends Controller
             ]);
         }
 
+        $encodedValues = json_encode($form['attribute_values'], JSON_THROW_ON_ERROR | JSON_UNESCAPED_UNICODE);
+        $duplicate     = $this->repo()->findDuplicate((int) $form['customer_type_id'], $encodedValues);
+
+        if ($duplicate !== null) {
+            $model->form           = $form;
+            $model->errors['_duplicate'] = [
+                'A customer with these exact values already exists: <a href="/customers/' . (int) $duplicate['id'] . '/edit">Customer #' . (int) $duplicate['id'] . ' (' . htmlspecialchars((string) $duplicate['customer_type_name'], ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8') . ')</a>.',
+            ];
+
+            return $this->view('customers.create', [
+                'model'     => $model,
+                'pageTitle' => $model->title,
+            ]);
+        }
+
         $customer                  = new Customer();
         $customer->customerTypeId  = (int) $form['customer_type_id'];
         $customer->attributeValues = $form['attribute_values'];
@@ -159,6 +174,21 @@ class CustomerController extends Controller
             ]);
         }
 
+        $encodedValues = json_encode($form['attribute_values'], JSON_THROW_ON_ERROR | JSON_UNESCAPED_UNICODE);
+        $duplicate     = $this->repo()->findDuplicate((int) $form['customer_type_id'], $encodedValues, (int) $id);
+
+        if ($duplicate !== null) {
+            $model->form           = $form;
+            $model->errors['_duplicate'] = [
+                'These values are identical to an existing customer: <a href="/customers/' . (int) $duplicate['id'] . '/edit">Customer #' . (int) $duplicate['id'] . ' (' . htmlspecialchars((string) $duplicate['customer_type_name'], ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8') . ')</a>.',
+            ];
+
+            return $this->view('customers.edit', [
+                'model'     => $model,
+                'pageTitle' => $model->title,
+            ]);
+        }
+
         $customer                  = new Customer();
         $customer->id              = (int) $id;
         $customer->customerTypeId  = (int) $form['customer_type_id'];
diff --git a/app/Repositories/CustomerRepository.php b/app/Repositories/CustomerRepository.php
index bf263c4..6a85055 100644
--- a/app/Repositories/CustomerRepository.php
+++ b/app/Repositories/CustomerRepository.php
@@ -88,6 +88,24 @@ class CustomerRepository extends Repository
         );
     }
 
+    /** Returns an existing customer with the same type and attribute values, excluding $excludeId (use for update). */
+    public function findDuplicate(int $typeId, string $attributeValuesJson, int $excludeId = 0): ?array
+    {
+        $sql    = 'SELECT TOP (1) c.id, ct.name AS customer_type_name
+                   FROM customer c
+                   INNER JOIN customer_type ct ON c.customer_type_id = ct.id
+                   WHERE c.customer_type_id = :type_id
+                     AND c.attribute_values  = :attribute_values';
+        $params = ['type_id' => $typeId, 'attribute_values' => $attributeValuesJson];
+
+        if ($excludeId > 0) {
+            $sql   .= ' AND c.id != :exclude_id';
+            $params['exclude_id'] = $excludeId;
+        }
+
+        return $this->database->first($sql, $params);
+    }
+
     /** Used after INSERT to recover the generated id for audit logging. */
     public function findLatestByType(int $typeId): ?array
     {
diff --git a/app/Views/customers/create.php b/app/Views/customers/create.php
index c8d38c0..f2ad9d8 100644
--- a/app/Views/customers/create.php
+++ b/app/Views/customers/create.php
@@ -26,6 +26,10 @@ window.__initialCtVals = <?= json_encode($model->form['attribute_values'], JSON_
             <div class="alert alert-error"><?= e($model->errors['_token'][0]) ?></div>
         <?php endif; ?>
 
+        <?php if (isset($model->errors['_duplicate'])): ?>
+            <div class="alert alert-error"><?= $model->errors['_duplicate'][0] ?></div>
+        <?php endif; ?>
+
         <form method="post" action="/customers" class="ct-form" novalidate>
             <?= csrf_field() ?>
 
diff --git a/app/Views/customers/edit.php b/app/Views/customers/edit.php
index 0e73b26..a50c9d3 100644
--- a/app/Views/customers/edit.php
+++ b/app/Views/customers/edit.php
@@ -27,6 +27,10 @@ window.__initialCtVals = <?= json_encode($model->form['attribute_values'], JSON_
             <div class="alert alert-error"><?= e($model->errors['_token'][0]) ?></div>
         <?php endif; ?>
 
+        <?php if (isset($model->errors['_duplicate'])): ?>
+            <div class="alert alert-error"><?= $model->errors['_duplicate'][0] ?></div>
+        <?php endif; ?>
+
         <form method="post" action="/customers/<?= e((string) $customerId) ?>/update" class="ct-form" novalidate>
             <?= csrf_field() ?>