Ensure part uniqueness

Author: gonzalocasas

src/compas/datastructures/assembly/assembly.py

@@ -86,6 +86,8 @@ def add_part(self, part, key=None, **kwargs):
             The identifier of the part in the current assembly graph.
 
         """
+        if part.guid in self._parts:
+            raise AssemblyError('Part already added to the assembly')
         key = self.graph.add_node(key=key, part=part, **kwargs)
         part.key = key
         self._parts[part.guid] = part