Own plugin calling function twice

I have created an own plugin. I added it to a wordpress page with following action:

add_shortcode("myplugin", "MyFunction");

I am posting some information via form/POST to this plugin. Unfortunately one important function (calling external URL/API via CURL) is processed twice every time I am posting this information in function1().

Do you have any idea why/how to avoid this?

function MyFunction($atts) {

          $content = "Content...";
          // [...]
		  // switching tasks by GET/POST-parameters
		  switch ($task) {
          	case "task1":
          		$content = function1($value1, $value2);
          		break;
          default: 
          		$content = showform();
          }
		  
		  return $content;
}