Single Sign On in ASP.NET

Question

Hi all,

I want to know about security issues of Single Sign On.

1. Single Sign On means - The user just have to type the URL in the address bar
 e.g. http://1.2.3.4/abcd.aspx and hit "Enter".

2. The website directly redirects to the user account.

3. The website bypasses the Login page (login credentials).

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Question-1.
How to implement this on existing website with 155 users?

Question-2.
If Single Sign On is implemented, is this not risky?
E.g., IF a user sits on others machine and hits the URL, he will be able to see the confidential data of others.
How to avoid this ?
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