sessions in dotnet

Question

hy i have website in which i used to collect user details like photo,sign and education details.i have implemented session on my website but still there is some flaw which i am not able t understand if two user fill the form from same pc then sometime 2 user session overrides first user details. i am flushing session on logout from c# code as i am updating details on basis of value in session . and session have unique id in it