I can reproduce this error like so;
Login on FireFox (Test1):
f1984095@nwytg.net
testtest
Login on Chrome (Test2):
wkl11047@awsoo.com
testtest
Then I refresh FireFox a couple of times, followed by a couple of refreshes in Chrome and then FireFox again.
After a few seconds on Firefox I am also logged in as Test2.
This isn't default laravel behaviour and without code I can't really tell what is wrong.. But I would really re-check the login stuff..
Both the code as the session files that get written depeding on which session driver you use.
But also check for stupid gotcha's like someone using methods like Auth::logingUsingId($SomeWeirdValueThatIsLastLoggedInUser);
My answer is quite vague, but so is your problem description.