Session swapping in asp.net 2.0 appplication

Question

Hi, Actually i have a web application developed using asp.net, which allocates a document per user to work on and the document follow a workflow until its reached the end of the process. From the last few weeks, several cases were reported where user ABC gets a document which is already allocated to user XYZ was working with.    User ABC was not supposed to get the document. As per log details in the database, document was never allocated to user ABC and record in database was well updated and document was checkout on user XYZ. Logically speaking, user ABC should not get a document that is already checkout\allocated to another user, the code logic is correct. And I was another to prove in in my development environment. 

Application is accessed by users through their Terminal server. Terminal server can be accessed remotely by several users. 

Is that a case of session swapping? or an issue raised because I am using Response.Redirect("Login.aspx")????

Also notice that when calling a procedure to get an object for an ID 123, in return i get object for ID 456. Object ID 456 was actually being used  by another user session! is that also a case of session swapping?

Please help, i really a solution to this problem since its occurring quite often around 27times per day on around 600 documents being processed daily by 20 users log in on a Terminal server