diff --git a/reclass/datatypes/parameters.py b/reclass/datatypes/parameters.py
index 9f5b9aa..37419fc 100644
--- a/reclass/datatypes/parameters.py
+++ b/reclass/datatypes/parameters.py
@@ -70,6 +70,13 @@
         return self._base.copy()
 
     def _update_scalar(self, cur, new, path):
+        if isinstance(cur, RefValue) and path in self._occurrences:
+            # If the current value already holds a RefValue, we better forget
+            # the occurrence, or else interpolate() will later overwrite
+            # unconditionally. If the new value is a RefValue, the occurrence
+            # will be added again further on
+            del self._occurrences[path]
+
         if self.delimiter is None or not isinstance(new, (types.StringTypes,
                                                           RefValue)):
             # either there is no delimiter defined (and hence no references
diff --git a/reclass/datatypes/tests/test_parameters.py b/reclass/datatypes/tests/test_parameters.py
index 18fc5dd..f58056d 100644
--- a/reclass/datatypes/tests/test_parameters.py
+++ b/reclass/datatypes/tests/test_parameters.py
@@ -127,6 +127,12 @@
             self.assertIn(mock.call(key), b1.get.call_args_list)
             self.assertIn(mock.call(key, value), b1.__setitem__.call_args_list)
 
+    def test_stray_occurrence_overwrites_during_interpolation(self):
+        p1 = Parameters({'r' : mock.sentinel.ref, 'b': '${r}'})
+        p2 = Parameters({'b' : mock.sentinel.goal})
+        p1.merge(p2)
+        p1.interpolate()
+        self.assertEqual(p1.as_dict()['b'], mock.sentinel.goal)
 
 class TestParametersNoMock(unittest.TestCase):