[Editor] Disable enum property items with duplicate values

This helps with confusion over how selecting a key with a duplicate
value won't be selected as only the first entry with a particular value will
be selected.

editor/editor_properties.cpp

@@ -688,14 +688,21 @@ void EditorPropertyEnum::update_property() {
 
 void EditorPropertyEnum::setup(const Vector<String> &p_options) {
 	options->clear();
+	RBSet<int64_t> values;
 	int64_t current_val = 0;
 	for (int i = 0; i < p_options.size(); i++) {
 		Vector<String> text_split = p_options[i].split(":");
 		if (text_split.size() != 1) {
 			current_val = text_split[1].to_int();
 		}
-		options->add_item(text_split[0]);
+		if (values.has(current_val)) {
+			options->add_item(vformat(TTR("%s (duplicate)"), text_split[0]));
+			options->set_item_disabled(i, true);
+		} else {
+			options->add_item(text_split[0]);
+		}
 		options->set_item_metadata(i, current_val);
+		values.insert(current_val);
 		current_val += 1;
 	}
 }