How to reproduce "Multiuser Concurrency Problems" in testing environment with limited resources

Scenario:

Many users signup/login at the same time -> Sometimes it might happen 'user 1' login but details of 'user 2' is displayed on live server. Chance of this to happen is very rare. But need to reproduce it in testing environment so that we can go into root cause & correct it.

We have limited Testing resources , which are tools/Methods we should follow to replicate the issue.