Sounds like a problem with the IPS forum’s handler that handles cURL requests. I anticpate that it uses a library and the library cannot handle two parameters of the same key in the URL?
Try the equivalent request to the users endpoint for helix with your code logic and see if the bug occurs again.
https://api.twitch.tv/helix/users?login=userlogin&login=userlogin2
But with valid users and see if you get two users back with your IPS call.