Magento - users can see other data

We have a Magento store, and sometimes when users log into it, it is authenticated using the elses user information.

When a user logs into my account, they can see the details of the order of another customer.

I found a forum that said to activate Validate HTTP_USER_AGENT and check the REMOTE_ADDR values ​​in the session check settings, but we still see the problem.

Does anyone have any ideas on what might cause this problem?

Thanks in advance for your help.

George