freeze_variable: don't add to collection if not originally trainable

tensorpack/tfutils/varreplace.py

@@ -110,7 +110,9 @@ def freeze_variables(stop_gradient=True, skip_collection=False):
         if skip_collection:
             kwargs['trainable'] = False
         v = getter(*args, **kwargs)
-        if skip_collection:
+        # do not perform unnecessary changes if it's not originally trainable
+        # otherwise the variable may get added to MODEL_VARIABLES twice
+        if trainable and skip_collection:
             tf.add_to_collection(tf.GraphKeys.MODEL_VARIABLES, v)
         if trainable and stop_gradient:
             v = tf.stop_gradient(v, name='freezed_' + name)