Both true and false got from checkbox if it is checked

In my MVC view I have a form with method GET. In that form I have several fields which works fine. After adding a checkbox I found strange behavior. If that checkbox is unchecked I have myCheckbox=false in GET parameter, but when I check the checkbox I have myCheckbox=true&myCheckbox=false.

My view:

...
@using (Html.BeginForm("MyAction", "MyController", FormMethod.Get)) { 
<fieldset>
    SomeInput: @Html.TextBox("someInput") <br />
    ...
    MyCheckbox: @Html.CheckBox("myCheckbox") <br />
</fieldset>
}
...

With value data in SomeInput and unchecked checkbox I'm getting url:

...?someInput=data&myCheckbox=false

and with checked checkbox I'm getting:

...?someInput=data&myCheckbox=true&myCheckbox=false