how to remove an eventhandler from a collection?

Ciao to everybody

I'm adding an action on selection of objects with this:  

Rhino.RhinoDoc.SelectObjects += myHandler 

My problem is that everytime that I run the script the myHandler sub is added to the collection of the eventhandlers.

Let me do an example...

Rhino.RhinoDoc.SelectObjects += myHandler 

private void myHandler(object sender, RhinoObjectSelectionEventArgs e)
{
Rhino.RhinoApp.WriteLine("ciao");

}

first time that I run the script the command line shows the word "ciao" each time that I select something

second time the word "ciao" is showed two times. etc

I tried  with this Rhino.RhinoDoc.SelectObjects -= myHandler but it doesn't work.

any idea?

ciao

Vito