diff --git a/wlauto/tests/test_config.py b/wlauto/tests/test_config.py
index a7e9a5d2..0587ddb5 100644
--- a/wlauto/tests/test_config.py
+++ b/wlauto/tests/test_config.py
@@ -22,7 +22,7 @@ from unittest import TestCase
 from nose.tools import assert_equal, assert_in, raises
 
 from wlauto.core.bootstrap import ConfigLoader
-from wlauto.core.agenda import AgendaWorkloadEntry, AgendaGlobalEntry
+from wlauto.core.agenda import AgendaWorkloadEntry, AgendaGlobalEntry, Agenda
 from wlauto.core.configuration import RunConfiguration
 from wlauto.exceptions import ConfigError
 
@@ -33,24 +33,35 @@ BAD_CONFIG_TEXT = """device = 'TEST
 device_config = 'TEST-CONFIG'"""
 
 
+LIST_PARAMS_AGENDA_TEXT = """
+config:
+    instrumentation: [list_params]
+    list_params:
+        param: [0.1, 0.1, 0.1]
+workloads:
+    - dhrystone
+"""
+
+
 class MockExtensionLoader(object):
 
     def __init__(self):
         self.aliases = {}
         self.global_param_aliases = {}
-        self.extensions = {}
+        self.extensions = {
+            'defaults_workload': DefaultsWorkload(),
+            'list_params': ListParamstrument(),
+        }
 
     def get_extension_class(self, name, kind=None):  # pylint: disable=unused-argument
-        if name == 'defaults_workload':
-            return DefaultsWorkload()
-        else:
-            return NamedMock(name)
+        return self.extensions.get(name, NamedMock(name))
 
     def resolve_alias(self, name):
         return name, {}
 
     def get_default_config(self, name):  # pylint: disable=unused-argument
-        return {}
+        ec = self.get_extension_class(name)
+        return {p.name: p.default for p in ec.parameters}
 
     def has_extension(self, name):
         return name in self.aliases or name in self.extensions
@@ -88,6 +99,14 @@ class DefaultsWorkload(object):
         self.parameters[0].default = [1, 2]
 
 
+class ListParamstrument(object):
+
+    def __init__(self):
+        self.name = 'list_params'
+        self.parameters = [NamedMock('param')]
+        self.parameters[0].default = []
+
+
 class ConfigLoaderTest(TestCase):
 
     def setUp(self):
@@ -142,6 +161,12 @@ class ConfigTest(TestCase):
         spec = self.config.workload_specs[0]
         assert_equal(spec.workload_parameters, {'param': [3]})
 
+    def test_exetension_params_lists(self):
+        a = Agenda(LIST_PARAMS_AGENDA_TEXT)
+        self.config.set_agenda(a)
+        self.config.finalize()
+        assert_equal(self.config.instrumentation['list_params']['param'], [0.1, 0.1, 0.1])
+
     def test_global_instrumentation(self):
         self.config.load_config({'instrumentation': ['global_instrument']})
         ws = AgendaWorkloadEntry(id='a', iterations=1, name='linpack', instrumentation=['local_instrument'])
diff --git a/wlauto/utils/misc.py b/wlauto/utils/misc.py
index 8e93ece5..1eed15d8 100644
--- a/wlauto/utils/misc.py
+++ b/wlauto/utils/misc.py
@@ -313,7 +313,23 @@ def merge_lists(*args, **kwargs):
 
 
 def _merge_two_lists(base, other, duplicates='all', dict_type=dict):  # pylint: disable=R0912
-    """Merge lists, normalizing their entries."""
+    """
+    Merge lists, normalizing their entries.
+
+    parameters:
+
+        :base, other: the two lists to be merged. ``other`` will be merged on
+                      top of base.
+        :duplicates: Indicates the strategy of handling entries that appear
+                     in both lists. ``all`` will keep occurrences from both
+                     lists; ``first`` will only keep occurrences from
+                     ``base``; ``last`` will only keep occurrences from
+                     ``other``;
+
+                     .. note:: duplicate entries that appear in the *same* list
+                               will never be removed.
+
+    """
     if duplicates == 'all':
         merged_list = []
         for v in normalize(base, dict_type) + normalize(other, dict_type):
@@ -321,20 +337,21 @@ def _merge_two_lists(base, other, duplicates='all', dict_type=dict):  # pylint:
                 merged_list.append(v)
         return merged_list
     elif duplicates == 'first':
-        merged_list = []
-        for v in normalize(base + other, dict_type):
+        base_norm = normalize(base, dict_type)
+        merged_list = normalize(base, dict_type)
+        for v in normalize(other, dict_type):
             if not _check_remove_item(merged_list, v):
-                if v not in merged_list:
-                    merged_list.append(v)
+                if v not in base_norm:
+                    merged_list.append(v)  # pylint: disable=no-member
         return merged_list
     elif duplicates == 'last':
+        other_norm = normalize(other, dict_type)
         merged_list = []
-        for v in normalize(base + other, dict_type):
+        for v in normalize(base, dict_type):
             if not _check_remove_item(merged_list, v):
-                if v in merged_list:
-                    del merged_list[merged_list.index(v)]
-                merged_list.append(v)
-        return merged_list
+                if v not in other_norm:
+                    merged_list.append(v)
+        return merged_list + other_norm
     else:
         raise ValueError('Unexpected value for list duplcates argument: {}. '.format(duplicates) +
                          'Must be in {"all", "first", "last"}.')