ensure unique recipients

app/views/submissions/_error.html.erb

@@ -0,0 +1,7 @@
+<div id="submitters_error">
+  <% if local_assigns[:error] %>
+    <div class="text-center text-red-500">
+      <%= local_assigns[:error] %>
+    </div>
+  <% end %>
+</div>

app/views/submissions/new.html.erb

@@ -28,6 +28,7 @@
     <div id="list" class="hidden">
       <%= render 'list_form', template: @template %>
     </div>
+    <%= render 'submissions/error' %>
   </div>
   <%= content_for(:modal_extra) %>
 <% end %>