You probably know how hard it is to purposefully cause a concurrency issue so you can test your code in these edge scenarios. I ran into this issue not too long ago and found that my favorite web service tester Postman doesn’t support sending multiple messages concurrently. I’m sure there are plenty of tools out there that do a great job, but for the simplest possible way you can use a combination of cURL and scripting. Here is an example using Windows, cURL and cmd script.
This is very simple. In fact, too simple to handle anything other than debug. But hey, I’m a developer! ;)