Thanks for your answer, but I must tell you that following the letter to your recommendations, yet detects me like simultaneous sessions for different users in different PCs from my internal network.
Also when it detects a "Connection simultaneous " sends several notices by email.
Please if you can help me about it I thank you in advance.
This seems unintended behavior indeed. With the above configuration, you should have:
When a single account logs in from 2 computers or from 2 browsers similarly (or one in regular mode and one in private mode, session being stored in cookie), without "returning only" checked, the 2nd user should get logged off (otherwise, the 1st one will trigger detection). An email should be sent each time a detection is triggered, that is each time a new connection is attempted in your case, or each time a returning user logs in with "returning only" checked.
I guess I should do some tests to see if for some reason the rule stopped working correctly.
I'll get back to you when I know more about it, and if it's a bug I'll send you a patch.