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In terms of brand reputation management, I really wonder why you sign up with your website name to then post about a considerable problem on that very website and about customers complaining about this serious issue. But that aside…
The issue is related to session management. You don’t give ANY information about your server specs, hosting environment, session settings etc. so it’s nigh impossible to actually give good instructions on how to remedy this, but it’s usually a case of poor, shared hosting and the sessions getting mixed up. There are plenty of threads in this forum about “seeing other customer’s details”. A Google search will probably get you to the answers instantly.