diff --git a/bw_timex/helper_classes.py b/bw_timex/helper_classes.py
index 564b2f7..36cc8b6 100644
--- a/bw_timex/helper_classes.py
+++ b/bw_timex/helper_classes.py
@@ -129,6 +129,10 @@ def add(self, process_time_tuple, unique_id=None):
             return self[process_time_tuple]
 
         if unique_id is not None:
+            if unique_id in self.values():
+                raise ValueError(
+                    f"Unique ID {unique_id} is already assigned to another process_time_tuple."
+                )
             self[process_time_tuple] = unique_id
             return unique_id