diff --git a/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php b/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php
--- a/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php
+++ b/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php
@@ -251,9 +251,8 @@
           pht(
             'Repository "%s" exists on more than one device, but no device '.
             'has any repository version information. Phabricator can not '.
-            'guess which copy of the existing data is authoritative. Remove '.
-            'all but one device from service to mark the remaining device '.
-            'as the authority.',
+            'guess which copy of the existing data is authoritative. Promote '.
+            'a device or see "Ambigous Leaders" in the documentation.',
             $repository->getDisplayName()));
       }
 
diff --git a/src/applications/repository/management/PhabricatorRepositoryManagementClusterizeWorkflow.php b/src/applications/repository/management/PhabricatorRepositoryManagementClusterizeWorkflow.php
--- a/src/applications/repository/management/PhabricatorRepositoryManagementClusterizeWorkflow.php
+++ b/src/applications/repository/management/PhabricatorRepositoryManagementClusterizeWorkflow.php
@@ -61,6 +61,7 @@
           array(
             AlmanacClusterRepositoryServiceType::SERVICETYPE,
           ))
+        ->needBindings(true)
         ->executeOne();
       if (!$service) {
         throw new PhutilArgumentUsageException(
@@ -70,9 +71,41 @@
       }
     }
 
-
     if ($service) {
       $service_phid = $service->getPHID();
+
+      $bindings = $service->getActiveBindings();
+
+      $unique_devices = array();
+      foreach ($bindings as $binding) {
+        $unique_devices[$binding->getDevicePHID()] = $binding->getDevice();
+      }
+
+      if (count($unique_devices) > 1) {
+        $device_names = mpull($unique_devices, 'getName');
+
+        echo id(new PhutilConsoleBlock())
+          ->addParagraph(
+            pht(
+              'Service "%s" is actively bound to more than one device (%s).',
+              $service_name,
+              implode(', ', $device_names)))
+          ->addParagraph(
+            pht(
+              'If you clusterize a repository onto this service it may be '.
+              'unclear which devices have up-to-date copies of the '.
+              'repository. If so, leader/follower ambiguity will freeze the '.
+              'repository. You may need to manually promote a device to '.
+              'unfreeze it. See "Ambiguous Leaders" in the documentation '.
+              'for discussion.'))
+          ->drawConsoleString();
+
+        $prompt = pht('Continue anyway?');
+        if (!phutil_console_confirm($prompt)) {
+          throw new PhutilArgumentUsageException(
+            pht('User aborted the workflow.'));
+        }
+      }
     } else {
       $service_phid = null;
     }
diff --git a/src/docs/user/cluster/cluster_repositories.diviner b/src/docs/user/cluster/cluster_repositories.diviner
--- a/src/docs/user/cluster/cluster_repositories.diviner
+++ b/src/docs/user/cluster/cluster_repositories.diviner
@@ -422,17 +422,22 @@
 =================
 
 Repository clusters can also freeze if the leader devices are ambiguous. This
-can happen if you replace an entire cluster with new devices suddenly, or
-make a mistake with the `--demote` flag. This generally arises from some kind
-of operator error, like this:
+can happen if you replace an entire cluster with new devices suddenly, or make
+a mistake with the `--demote` flag. This may arise from some kind of operator
+error, like these:
 
   - Someone accidentally uses `bin/repository thaw ... --demote` to demote
     every device in a cluster.
   - Someone accidentally deletes all the version information for a repository
     from the database by making a mistake with a `DELETE` or `UPDATE` query.
-  - Someone accidentally disable all of the devices in a cluster, then add
+  - Someone accidentally disables all of the devices in a cluster, then adds
     entirely new ones before repositories can propagate.
 
+If you are moving repositories into cluster services, you can also reach this
+state if you use `clusterize` to associate a repository with a service that is
+bound to multiple active devices. In this case, Phabricator will not know which
+device or devices have up-to-date information.
+
 When Phabricator can not tell which device in a cluster is a leader, it freezes
 the cluster because it is possible that some devices have less data and others
 have more, and if it choses a leader arbitrarily it may destroy some data