saml:SP: Handle duplicate response.
saml:SP: Handle duplicate response.

In some cases, we may receive duplicate responses when the user uses
the back-button in the browser to go back to the HTTP-POST page. This
patch changes the behaviour to just redirect the user to the correct
page, instead of displaying a confusing error message.

Fixes issue 404.

simplesamlphp/saml2,sgomez/saml2,sitya/saml2,newmen/saml2,ArtemTeleshev/php-saml2,madmatt/saml2,cb8/saml2