diff --git a/cassis/cas.py b/cassis/cas.py
index b5140e3..2ca8668 100644
--- a/cassis/cas.py
+++ b/cassis/cas.py
@@ -645,7 +645,7 @@ def _find_all_fs(self, generate_missing_ids: bool = False) -> Iterable[FeatureSt
         ts = self.typesystem
         while openlist:
             fs = openlist.pop(0)
-            if generate_missing_ids:
+            if generate_missing_ids and fs.xmiID is None:
                 fs.xmiID = self._get_next_xmi_id()
             all_fs[fs.xmiID] = fs