diff --git a/masque/builder/pather.py b/masque/builder/pather.py
index cbf87f4..877cd6d 100644
--- a/masque/builder/pather.py
+++ b/masque/builder/pather.py
@@ -159,6 +159,16 @@ class Pather(PortList):
 
     def _route_contexts(self, portspecs: Sequence[str]) -> tuple[RoutePortContext, ...]:
         """Snapshot several ports in request order for bundle planning."""
+        if not portspecs:
+            raise BuildError('Routing requires at least one port')
+        seen: set[str] = set()
+        duplicates: set[str] = set()
+        for portspec in portspecs:
+            if portspec in seen:
+                duplicates.add(portspec)
+            seen.add(portspec)
+        if duplicates:
+            raise BuildError(f'Routing port names must be unique; got duplicates: {sorted(duplicates)}')
         return tuple(self._route_context(portspec) for portspec in portspecs)
 
     @property