Are you opening 2 browser windows on the same pc? If so I think they might end up shareing the same session cookie. I've never been too sure on this - it might depend on the browser implementation.
I've also seen this session confusion happen from seperate pcs where the requests were routed through a proxy that did some caching. User 1 requested a personalized which displayed correctly, as did user 2, but then user 1 got user 2's page. Had to get them to set their IE browser so that it didn't use the proxy for requests to the application.
I agree with the last user about sharing the same session stuff, but how are you opening your browser? Are you clicking on the browser, then hitting control-N for another browser to open? If so, it will definetly share same session, but if you open two separate browser instances, there should be no conflicting data. If this doesn't help, could you post the code and we could try to replicate the problem? Thanks.