Ensure node are present only once in the desugared AST

test/yarp/desugar_visitor_test.rb

@@ -53,6 +53,24 @@ def ast_inspect(node)
   def assert_desugars(expected, source)
     ast = YARP.parse(source).value.accept(YARP::DesugarVisitor.new)
     assert_equal expected, ast_inspect(ast.statements.body.last)
+
+    # Ensure every node is only present once in the AST.
+    # If the same node is present twice it would most likely indicate it is executed twice, which is invalid semantically.
+    # This also acts as a sanity check that Node#child_nodes returns only nodes or nil (which caught a couple bugs).
+    all_nodes = {}.compare_by_identity
+    visitor = Class.new(YARP::Visitor) do
+      define_method :visit do |node|
+        if node
+          if all_nodes.include?(node)
+            raise "#{node.inspect} is present multiple times in the desugared AST and likely executed multiple times"
+          else
+            all_nodes[node] = true
+          end
+        end
+        super(node)
+      end
+    end
+    ast.accept(visitor.new)
   end
 
   def assert_not_desugared(source, reason)