diff --git a/stix2/environment.py b/stix2/environment.py
index a3d79bc..f855755 100644
--- a/stix2/environment.py
+++ b/stix2/environment.py
@@ -1,3 +1,5 @@
+import copy
+
 
 class ObjectFactory(object):
     """Object Factory
@@ -44,7 +46,7 @@ class ObjectFactory(object):
     def create(self, cls, **kwargs):
         # Use self.defaults as the base, but update with any explicit args
         # provided by the user.
-        properties = dict(**self._defaults)
+        properties = copy.deepcopy(self._defaults)
         if kwargs:
             if self._list_append:
                 # Append provided items to list properties instead of replacing them
@@ -55,7 +57,11 @@ class ObjectFactory(object):
                         continue
                     if not isinstance(properties[list_prop], list):
                         properties[list_prop] = [properties[list_prop]]
-                    properties[list_prop].append(kwarg_prop)
+
+                    if isinstance(kwarg_prop, list):
+                        properties[list_prop].extend(kwarg_prop)
+                    else:
+                        properties[list_prop].append(kwarg_prop)
 
             properties.update(**kwargs)
 
diff --git a/stix2/test/test_environment.py b/stix2/test/test_environment.py
index 9e5ab98..9be8101 100644
--- a/stix2/test/test_environment.py
+++ b/stix2/test/test_environment.py
@@ -62,10 +62,15 @@ def test_object_factory_list_append():
                                       description="Threat report from ACME")
     ext_ref2 = stix2.ExternalReference(source_name="Yet Another Threat Report",
                                        description="Threat report from YATR")
+    ext_ref3 = stix2.ExternalReference(source_name="Threat Report #3",
+                                       description="One more threat report")
     factory = stix2.ObjectFactory(external_references=ext_ref)
     ind = factory.create(stix2.Indicator, external_references=ext_ref2, **INDICATOR_KWARGS)
     assert ind.external_references[1].source_name == "Yet Another Threat Report"
 
+    ind = factory.create(stix2.Indicator, external_references=[ext_ref2, ext_ref3], **INDICATOR_KWARGS)
+    assert ind.external_references[2].source_name == "Threat Report #3"
+
 
 def test_object_factory_list_replace():
     ext_ref = stix2.ExternalReference(source_name="ACME Threat Intel",