This bug appears in the current early access release of JDK 8 (b126) and also in the previous one.

Minimal code to reproduce the issue:

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

public class Test extends Application
{
public static void main(String[] args)
{
launch(args);
}

@Override
public void start(Stage primaryStage) throws Exception
{
final ComboBox<String> cb = new ComboBox<String>();
cb.setEditable(true);
cb.setOnAction(new EventHandler<ActionEvent>()
{
int counter = 0;
@Override
public void handle(ActionEvent event)
{
System.out.println(counter++);
}
});
primaryStage.setScene(new Scene(cb));
primaryStage.show();
}
}

Run and type something text in the ComboBox text field, then press Enter. The OnAction event handler gets called twice (two numbers appear in the console).