Based on your conversation with the support agent, it seems they’re quite confident that you logged in from a device that other users have also used.
You mentioned accessing Rollbit from your school’s PCs, which could explain it.
Additionally, you said you used your girlfriend’s laptop, does she also have a Rollbit account?
Unfortunately, proving that you don’t have multiple accounts can be very challenging.
All these could be the reason why they are emanating that op has other accounts, and of course I wouldn't doubt for any reason because op could share his ref link to the girlfriend and friends in school to sign up and later went to access his account using same system, by then the system could automatically fish him out as someone who has several accounts on their site since they are all accessing the site using same computer or same network.