The example below adds the same TableColumn more than once: in case A, that's done across nested columns -> run and see a NPE produced in the sizing code, the real error is not indicated. For comparison, case B (un/comment the respective lines) adds the duplicate in the same level of nesting -> run and see the IllegalStateException, which is expected.

The example:

import java.util.Locale;
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.scene.control.cell.PropertyValueFactory;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class DuplicateTableColumnNPE extends Application {

    private Parent getContent() {
        TableView<Locale> table = new TableView<>(FXCollections.observableArrayList(
                Locale.getAvailableLocales()));
        TableColumn<Locale, String> country = new TableColumn<>("Country");
        TableColumn<Locale, String> countryCode = new TableColumn<>("Code");
        countryCode.setCellValueFactory(new PropertyValueFactory<>("country"));
        TableColumn<Locale, String> countryDisplay = new TableColumn<>("Name");
        countryDisplay.setCellValueFactory(new PropertyValueFactory<>("displayCountry"));
        // A: this throws NPE in auto-sizing code
        // incorrect exception
        // that is duplicates across nested headers are not detected
        country.getColumns().addAll(countryCode, countryDisplay);
        table.getColumns().addAll(countryCode);
        
        // B: this throws IllegalStateException - duplicate columns
        // correct exception
        // table.getColumns().addAll(countryCode, countryCode);
        
        BorderPane pane = new BorderPane(table);
        return pane;
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        primaryStage.setScene(new Scene(getContent(), 800, 400));
        primaryStage.show();
    }
    
    public static void main(String[] args) {
        launch(args);
    }

}