How does iOS app Badoo tracks second registrations?
iOS apps can use the DeviceCheck API to store two bits of persistent information per device on Apple servers. This data is not cleared when an application is uninstalled; some apps may use one or both of the bits to flag devices which have already taken advantage of a free trial, or which have been blocked for abuse.

This API cannot be used to track specific devices, as it is too small to store a unique identifier. (There are only four possible values which can be stored in two bits: 00, 01, 10, and 11.)

There is no way for you to clear this data, as it is stored on Apple servers.