currently writing a Booking clone application. I wanna fill it with users,I currently have 553m users from Facebook leak. The project will not be sold or anything,its just for show. Can I do it? Someone using the Application will not be able to see other users etc. But lets say for example a random guy tries to make an account after cloning and he tries his credentials,my system will not allow it if hes already fake "registered". Do i have any issue with that?
This isn't a matter of copyright. One cannot copyright a name and copyright does not protect personal data, although the related database right might well be relevant, depending on the country involved.
But the question is tagged GDPR. If the GDPR applies, that is if the app developer is in the EU or the UK, or if the service is targeted at the EU or UK and some of the users are in the EU or UK, there will be a problem with this. A User's name and other info are clearly Personal Data (PI) under the GDPR, and there must therefore be a lawful basis for the processing. If user named were obtained via a leak, there is no consent, and it is hard to see what basis could plausibly apply.