diff --git a/src/wireviz/DataClasses.py b/src/wireviz/DataClasses.py
index 52ff9830..44694174 100644
--- a/src/wireviz/DataClasses.py
+++ b/src/wireviz/DataClasses.py
@@ -51,6 +51,9 @@ def __post_init__(self):
         if not self.pinout:
             self.pinout = [''] * self.pincount
 
+        if len(self.pinnumbers) != len(set(self.pinnumbers)):
+            raise Exception('Pin numbers are not unique')
+
     def loop(self, from_pin, to_pin):
         self.loops.append((from_pin, to_pin))
         if self.hide_disconnected_pins: