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.control.TextField;
import javafx.scene.layout.Pane;
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 {
        Pane pane = new Pane();
        pane.setPrefHeight(200);
        pane.setPrefWidth(200);

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

        pane.getChildren().add(testedComboBox);

        VBox vb = new VBox();

        testedComboBox.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent t) {
                System.out.println(counter++);
                System.out.println(testedComboBox.getValue());
            }
        });

        TextField tf = new TextField("Some value");
        testedComboBox.valueProperty().bind(tf.textProperty());

        vb.getChildren().addAll(pane, tf);
        Scene scene = new Scene(vb, 400, 400);
        stage.setScene(scene);
        stage.show();
    }
}

To reproduce:
remove char by char string from text field.

I see output:
run:
0
Some value
1
Some valu
2
Some valu
3
Some val
4
Some val
5
Some va
6
Some va

You can see, that listener is called 2 times each time.