diff --git a/changes.d/6335.fix.md b/changes.d/6335.fix.md
new file mode 100644
index 00000000000..2057bae0f7c
--- /dev/null
+++ b/changes.d/6335.fix.md
@@ -0,0 +1 @@
+Fix an issue that could cause broadcasts made to multiple namespaces to fail.
diff --git a/cylc/flow/broadcast_mgr.py b/cylc/flow/broadcast_mgr.py
index 6cd007ee25a..b9329114302 100644
--- a/cylc/flow/broadcast_mgr.py
+++ b/cylc/flow/broadcast_mgr.py
@@ -280,8 +280,16 @@ def put_broadcast(
         bad_namespaces = []
 
         with self.lock:
-            for setting in settings:
-                for point_string in point_strings:
+            for setting in settings or []:
+                # Coerce setting to cylc runtime object,
+                # i.e. str to  DurationFloat.
+                coerced_setting = deepcopy(setting)
+                BroadcastConfigValidator().validate(
+                    coerced_setting,
+                    SPEC['runtime']['__MANY__'],
+                )
+
+                for point_string in point_strings or []:
                     # Standardise the point and check its validity.
                     bad_point = False
                     try:
@@ -292,26 +300,23 @@ def put_broadcast(
                             bad_point = True
                     if not bad_point and point_string not in self.broadcasts:
                         self.broadcasts[point_string] = {}
-                    for namespace in namespaces:
+                    for namespace in namespaces or []:
                         if namespace not in self.linearized_ancestors:
                             bad_namespaces.append(namespace)
                         elif not bad_point:
                             if namespace not in self.broadcasts[point_string]:
                                 self.broadcasts[point_string][namespace] = {}
+
                             # Keep saved/reported setting as workflow
-                            # config format.
+                            # config format:
                             modified_settings.append(
-                                (point_string, namespace, deepcopy(setting))
-                            )
-                            # Coerce setting to cylc runtime object,
-                            # i.e. str to  DurationFloat.
-                            BroadcastConfigValidator().validate(
-                                setting,
-                                SPEC['runtime']['__MANY__']
+                                (point_string, namespace, setting)
                             )
+
+                            # Apply the broadcast with the "coerced" format:
                             addict(
                                 self.broadcasts[point_string][namespace],
-                                setting
+                                coerced_setting,
                             )
 
         # Log the broadcast
diff --git a/cylc/flow/scripts/broadcast.py b/cylc/flow/scripts/broadcast.py
index c7e5d2a4f3b..2cf068309b7 100755
--- a/cylc/flow/scripts/broadcast.py
+++ b/cylc/flow/scripts/broadcast.py
@@ -325,6 +325,10 @@ async def run(options: 'Values', workflow_id):
     """Implement cylc broadcast."""
     pclient = get_client(workflow_id, timeout=options.comms_timeout)
 
+    # remove any duplicate namespaces
+    # see https://github.com/cylc/cylc-flow/issues/6334
+    namespaces = list(set(options.namespaces))
+
     ret: Dict[str, Any] = {
         'stdout': [],
         'stderr': [],
@@ -337,7 +341,7 @@ async def run(options: 'Values', workflow_id):
             'wFlows': [workflow_id],
             'bMode': 'Set',
             'cPoints': options.point_strings,
-            'nSpaces': options.namespaces,
+            'nSpaces': namespaces,
             'bSettings': options.settings,
             'bCutoff': options.expire,
         }
@@ -382,7 +386,6 @@ async def run(options: 'Values', workflow_id):
         mutation_kwargs['variables']['bMode'] = 'Expire'
 
     # implement namespace and cycle point defaults here
-    namespaces = options.namespaces
     if not namespaces:
         namespaces = ["root"]
     point_strings = options.point_strings
diff --git a/tests/integration/scripts/test_broadcast.py b/tests/integration/scripts/test_broadcast.py
index 67a79448041..cad23cdf386 100644
--- a/tests/integration/scripts/test_broadcast.py
+++ b/tests/integration/scripts/test_broadcast.py
@@ -21,7 +21,7 @@
 BroadcastOptions = Options(get_option_parser())
 
 
-async def test_broadcast_multi(
+async def test_broadcast_multi_workflow(
     one_conf,
     flow,
     scheduler,
@@ -77,3 +77,49 @@ async def test_broadcast_multi(
                     ' settings are not compatible with the workflow'
                 ) in out
                 assert err == ''
+
+
+async def test_broadcast_multi_namespace(
+    flow,
+    scheduler,
+    start,
+):
+    """Test a multi-namespace broadcast command.
+
+    See https://github.com/cylc/cylc-flow/issues/6334
+    """
+    id_ = flow(
+        {
+            'scheduling': {
+                'graph': {'R1': 'a & b & c & fin'},
+            },
+            'runtime': {
+                'root': {'execution time limit': 'PT1S'},
+                'VOWELS': {'execution time limit': 'PT2S'},
+                'CONSONANTS': {'execution time limit': 'PT3S'},
+                'a': {'inherit': 'VOWELS'},
+                'b': {'inherit': 'CONSONANTS'},
+                'c': {'inherit': 'CONSONANTS'},
+            },
+        }
+    )
+    schd = scheduler(id_)
+
+    async with start(schd):
+        # issue a broadcast to multiple namespaces
+        rets = await _main(
+            BroadcastOptions(
+                settings=['execution time limit = PT5S'],
+                namespaces=['root', 'VOWELS', 'CONSONANTS'],
+            ),
+            schd.workflow,
+        )
+
+        # the broadcast should succeed
+        assert list(rets.values()) == [True]
+
+        # each task should now have the new "execution time limit"
+        for task in ['a', 'b', 'c', 'fin']:
+            assert schd.broadcast_mgr.get_broadcast(
+                schd.tokens.duplicate(cycle='1', task=task)
+            ) == {'execution time limit': 5.0}