Setting the AspNetAntiForgeryOptions.SameSiteAttributeValue causes undefined behavior

Description

Steps to reproduce

1. Make sure latest the Microsoft patch is installed (kb4533094).
2. Add the following code:

   context.Services.Configure<AspNetAntiForgeryOptions>(options =>

   {

   options.EnableSameOriginCheck = true;

   options.SameSiteAttributeValue = SameSiteType.Strict;

   });

3. Enable the default cookie policy in the web.config file.

   <httpCookies sameSite="None" />

4. Go to login page in new browser (without any cookies)

Expected:
SameSite is set to Strict (no duplicates)

Actual:
SameSite appears twice
__epiXSRF=TiWREKhLeuFRJ+ZqgM6H0egChaVor+uqoPWuYm2+MBk=; path=/; SameSite=Strict; HttpOnly; SameSite=None