It usually is caused by that the riders entry account had duplicates of the same horse or riders. The internal ID of entry is used to determine if it is the same horse or rider.
And some riders have created them self twice, and then you have a payer and a rider (which is 2 persons, but one is responsible for all the payments). You can see this in the economy tab, with a notification starts are being payed by ....
Hope this clears it up, it is a bit of a inconvenience when the entry accounts of the riders are setup incorrectly.
