private double clickTime = 0f;
    ...
    if (GUILayout.Button("TestButton"))
    {
        if(EditorApplication.timeSinceStartup  - clickTime < 0.3f)
        {
            OnDoubleClick(pair.Value);
        }
        else
        {
            OnClick(pair.Value);
        }
        clickTime = EditorApplication.timeSinceStartup;
    }