So actually this wont be a problem. Basically if they do this then each account that they create will only have very few published predictions. So it will be li

So actually this wont be a problem. Basically if they do this then each account that they create will only have very few published predictions. So it will be like a coin which comes up head twice after two tosses. The public will not trust such accounts with low number of correct 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.