--- a/testing/taskcluster/mach_commands.py
+++ b/testing/taskcluster/mach_commands.py
@@ -360,25 +360,25 @@ class Graph(object):
         with open(routes_file) as f:
             contents = json.load(f)
             json_routes = contents['routes']
             # TODO: Nightly and/or l10n routes
 
         # Task graph we are generating for taskcluster...
         graph = {
             'tasks': [],
-            'scopes': []
+            'scopes': set(),
         }
 
         if params['revision_hash']:
             for env in routes_transform.TREEHERDER_ROUTES:
                 route = 'queue:route:{}.{}'.format(
                             routes_transform.TREEHERDER_ROUTES[env],
                             treeherder_route)
-                graph['scopes'].append(route)
+                graph['scopes'].add(route)
 
         graph['metadata'] = {
             'source': 'http://todo.com/what/goes/here',
             'owner': params['owner'],
             # TODO: Add full mach commands to this example?
             'description': 'Task graph generated via ./mach taskcluster-graph',
             'name': 'task graph local'
         }
@@ -436,20 +436,20 @@ class Graph(object):
                 if route.startswith('index.gecko.v2') and route in all_routes:
                     raise Exception("Error: route '%s' is in use by multiple tasks: '%s' and '%s'" % (
                         route,
                         build_task['task']['metadata']['name'],
                         all_routes[route],
                     ))
                 all_routes[route] = build_task['task']['metadata']['name']
 
-            graph['scopes'].append(define_task)
-            graph['scopes'].extend(build_task['task'].get('scopes', []))
+            graph['scopes'].add(define_task)
+            graph['scopes'] |= set(build_task['task'].get('scopes', []))
             route_scopes = map(lambda route: 'queue:route:' + route, build_task['task'].get('routes', []))
-            graph['scopes'].extend(route_scopes)
+            graph['scopes'] |= set(route_scopes)
 
             # Treeherder symbol configuration for the graph required for each
             # build so tests know which platform they belong to.
             build_treeherder_config = build_task['task']['extra']['treeherder']
 
             if 'machine' not in build_treeherder_config:
                 message = '({}), extra.treeherder.machine required for all builds'
                 raise ValueError(message.format(build['task']))
@@ -534,20 +534,20 @@ class Graph(object):
                         )
 
                     graph['tasks'].append(test_task)
 
                     define_task = DEFINE_TASK.format(
                         test_task['task']['workerType']
                     )
 
-                    graph['scopes'].append(define_task)
-                    graph['scopes'].extend(test_task['task'].get('scopes', []))
+                    graph['scopes'].add(define_task)
+                    graph['scopes'] |= set(test_task['task'].get('scopes', []))
 
-        graph['scopes'] = list(set(graph['scopes']))
+        graph['scopes'] = sorted(graph['scopes'])
 
         if params['print_names_only']:
             tIDs = defaultdict(list)
 
             def print_task(task, indent=0):
                 print('{}- {}'.format(' ' * indent, task['task']['metadata']['name']))
 
                 for child in tIDs[task['taskId']]:

--- a/testing/taskcluster/taskcluster_graph/image_builder.py
+++ b/testing/taskcluster/taskcluster_graph/image_builder.py
@@ -212,14 +212,14 @@ def normalize_image_details(graph, task,
 
     graph['tasks'].append(image_task);
     task['requires'].append(details['taskId'])
 
     define_task = DEFINE_TASK.format(
         image_task['task']['workerType']
     )
 
-    graph['scopes'].append(define_task)
-    graph['scopes'].extend(image_task['task'].get('scopes', []))
+    graph['scopes'].add(define_task)
+    graph['scopes'] |= set(image_task['task'].get('scopes', []))
     route_scopes = map(lambda route: 'queue:route:' + route, image_task['task'].get('routes', []))
-    graph['scopes'].extend(route_scopes)
+    graph['scopes'] |= set(route_scopes)
 
     details['required'] = True