Single-signon and custom security module

Hi,
I am using custom security module to control my web application security which also support windows authentication. I would like to implement single-singon feature as well. I was thinking to retrieve userName and domainName from WindowsIdentity.Name, to check against ActiveDirectory for authentication using DirectoryEntry.Guid.
However, I found that, DirectoryEntry required password in this case. How could I identify user by using userName and domainName only?
Beside, by using this approach might not secure as hacker can setup same user account with same user and domain name.
Any other way to authenticate user by using other property such as WindowsIdentity.Owner.Token ?
Please advice.
Thank you

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