You would need some concept of published identity, otherwise they could just create their predictions against multiple different identities each with e.g. 10 predictions.
To build trust you'd need to have a high count of revealed predictions in any given account.
That said people can try to game the system by uploading a very large number of predictions programmatically, so I think a rate throttling will have to be enforced. Any account can't make more than one prediction a day or something like that.
Also note that the issue you raised will also affect OP's implementation.
edit: and it's also your revenue.