You may run attached code:

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

/**
 * @author Alexander Kirov
 */
public class Issue4 extends Application {

    public static void main(String[] args) {
        launch(args);
    }
    final ComboBox<String> testedComboBox = new ComboBox<String>();
    static int counter = 0;

    @Override
    public void start(Stage stage) throws Exception {
        VBox pane = new VBox();
        pane.setPrefHeight(200);
        pane.setPrefWidth(200);
        testedComboBox.setEditable(true);

        testedComboBox.getItems().addAll("1", "2", "3");

        VBox vb = new VBox();

        pane.getChildren().addAll(testedComboBox);
        
        testedComboBox.setOnAction(new EventHandler<ActionEvent>(){

            public void handle(ActionEvent t) {
                System.out.println("Is called");
            }
        });
        
        vb.getChildren().addAll(pane);
        Scene scene = new Scene(vb, 400, 400);
        stage.setScene(scene);
        stage.show();
    }
}

Steps to reproduce:
type "a";
press Enter;

I see in output:
run:
Is called
Is called

But I expect only one call.

You may see issue on attached image.