diff --git a/resources/sql/autopatches/20180418.alamanc.interface.unique.php b/resources/sql/autopatches/20180418.alamanc.interface.unique.php
new file mode 100644
index 0000000000..0ad4fbea12
--- /dev/null
+++ b/resources/sql/autopatches/20180418.alamanc.interface.unique.php
@@ -0,0 +1,85 @@
+<?php
+
+$interface_table = new AlmanacInterface();
+$binding_table = new AlmanacBinding();
+$interface_conn = $interface_table->establishConnection('w');
+
+queryfx(
+  $interface_conn,
+  'LOCK TABLES %T WRITE, %T WRITE',
+  $interface_table->getTableName(),
+  $binding_table->getTableName());
+
+$seen = array();
+foreach (new LiskMigrationIterator($interface_table) as $interface) {
+  $device = $interface->getDevicePHID();
+  $network = $interface->getNetworkPHID();
+  $address = $interface->getAddress();
+  $port = $interface->getPort();
+  $key = "{$device}/{$network}/{$address}/{$port}";
+
+  // If this is the first copy of this row we've seen, mark it as seen and
+  // move on.
+  if (empty($seen[$key])) {
+    $seen[$key] = $interface->getID();
+    continue;
+  }
+
+  $survivor = queryfx_one(
+    $interface_conn,
+    'SELECT * FROM %T WHERE id = %d',
+    $interface_table->getTableName(),
+    $seen[$key]);
+
+  $bindings = queryfx_all(
+    $interface_conn,
+    'SELECT * FROM %T WHERE interfacePHID = %s',
+    $binding_table->getTableName(),
+    $interface->getPHID());
+
+  // Repoint bindings to the survivor.
+  foreach ($bindings as $binding) {
+    // Check if there's already a binding to the survivor.
+    $existing = queryfx_one(
+      $interface_conn,
+      'SELECT * FROM %T WHERE interfacePHID = %s and devicePHID = %s and '.
+      'servicePHID = %s',
+      $binding_table->getTableName(),
+      $survivor['phid'],
+      $binding['devicePHID'],
+      $binding['servicePHID']);
+
+    if (!$existing) {
+      // Reattach this binding to the survivor.
+      queryfx(
+        $interface_conn,
+        'UPDATE %T SET interfacePHID = %s WHERE id = %d',
+        $binding_table->getTableName(),
+        $survivor['phid'],
+        $binding['id']);
+    } else {
+      // Binding to survivor already exists. Remove this now-redundant binding.
+      queryfx(
+        $interface_conn,
+        'DELETE FROM %T WHERE id = %d',
+        $binding_table->getTableName(),
+        $binding['id']);
+    }
+  }
+
+  queryfx(
+    $interface_conn,
+    'DELETE FROM %T WHERE id = %d',
+    $interface_table->getTableName(),
+    $interface->getID());
+}
+
+queryfx(
+  $interface_conn,
+  'ALTER TABLE %T ADD UNIQUE KEY `key_unique` '.
+  '(devicePHID, networkPHID, address, port)',
+  $interface_table->getTableName());
+
+queryfx(
+  $interface_conn,
+  'UNLOCK TABLES');
diff --git a/src/applications/almanac/editor/AlmanacInterfaceEditor.php b/src/applications/almanac/editor/AlmanacInterfaceEditor.php
index 865402db36..771076efc9 100644
--- a/src/applications/almanac/editor/AlmanacInterfaceEditor.php
+++ b/src/applications/almanac/editor/AlmanacInterfaceEditor.php
@@ -15,4 +15,22 @@ final class AlmanacInterfaceEditor
     return pht('%s created %s.', $author, $object);
   }
 
+  protected function didCatchDuplicateKeyException(
+    PhabricatorLiskDAO $object,
+    array $xactions,
+    Exception $ex) {
+
+    $errors = array();
+
+    $errors[] = new PhabricatorApplicationTransactionValidationError(
+      null,
+      pht('Invalid'),
+      pht(
+        'Interfaces must have a unique combination of network, device, '.
+        'address, and port.'),
+      null);
+
+    throw new PhabricatorApplicationTransactionValidationException($errors);
+  }
+
 }
diff --git a/src/applications/almanac/storage/AlmanacInterface.php b/src/applications/almanac/storage/AlmanacInterface.php
index 4002651d08..5c7f65ddd1 100644
--- a/src/applications/almanac/storage/AlmanacInterface.php
+++ b/src/applications/almanac/storage/AlmanacInterface.php
@@ -35,6 +35,10 @@ final class AlmanacInterface
         'key_device' => array(
           'columns' => array('devicePHID'),
         ),
+        'key_unique' => array(
+          'columns' => array('devicePHID', 'networkPHID', 'address', 'port'),
+          'unique' => true,
+        ),
       ),
     ) + parent::getConfiguration();
   }