Sessions in Codeigniter gets changed during page load

Hi,

I am currently working on a CodeIgniter (v 3.1.9) project. It has a login page. 

When two different users (A and B) try to login from two different systems at same time, one of the user's session gets modified on loading. 

The loading takes a small amount of time and if the user A gets logged in first, it will be redirected to its own user homepage. Next, when user B completes its login, it will be redirected to user A's homepage which is strange. I also noticed that user B gets user A's session ID here. The same issue happens if the order of user login is changed, i.e if user B gets logged in first. The issue is noticed only when two users click the login button at the same time, otherwise it is fine.

Is it due to a bug related with CodeIgniter. Are there any workarounds to fix this issue?