Fixes #10929: Raise validation error when attempting to create a duplicate cable termination

docs/release-notes/version-3.3.md

@@ -15,6 +15,7 @@
 * [#10241](https://github.com/netbox-community/netbox/issues/10241) - Support referencing custom field related objects by attribute in addition to PK
 * [#10579](https://github.com/netbox-community/netbox/issues/10579) - Mark cable traces terminating to a provider network as complete
 * [#10721](https://github.com/netbox-community/netbox/issues/10721) - Disable ordering by custom object field columns
+* [#10929](https://github.com/netbox-community/netbox/issues/10929) - Raise validation error when attempting to create a duplicate cable termination
 * [#10938](https://github.com/netbox-community/netbox/issues/10938) - `render_field` template tag should respect `label` kwarg
 * [#10969](https://github.com/netbox-community/netbox/issues/10969) - Update cable paths ending at associated rear port when creating new front ports
 

netbox/dcim/models/cables.py

@@ -279,6 +279,17 @@ class CableTermination(models.Model):
     def clean(self):
         super().clean()
 
+        # Check for existing termination
+        existing_termination = CableTermination.objects.exclude(cable=self.cable).filter(
+            termination_type=self.termination_type,
+            termination_id=self.termination_id
+        ).first()
+        if existing_termination is not None:
+            raise ValidationError(
+                f"Duplicate termination found for {self.termination_type.app_label}.{self.termination_type.model} "
+                f"{self.termination_id}: cable {existing_termination.cable.pk}"
+            )
+
         # Validate interface type (if applicable)
         if self.termination_type.model == 'interface' and self.termination.type in NONCONNECTABLE_IFACE_TYPES:
             raise ValidationError(f"Cables cannot be terminated to {self.termination.get_type_display()} interfaces")