Several members submitted the same horse. What do I vote on?
Staff group duplicate submissions of the same horse, and the group reaches you as one item rather than as several. Each copy is a different picture of that horse, so the card opens as a gallery of every copy and asks which one you prefer.
Picking is not voting. It settles which copy your vote is cast against, and Back to options puts the gallery back so you can compare again. From there you vote exactly as you would on any other new horse: tap a thumb, then press submit.
Voting your pick up also votes the other copies down, so the votes gather on one of them. It still counts as one vote toward your daily votes and your reputation. Voting your pick down leaves the other copies alone.
Once you answer, the whole group leaves your queue, so you meet that horse once. Your own submission is never in the gallery, because you cannot vote for your own work.
The first copy to reach the approval threshold is the one added to the community database, and the rest close. A copy voted down to the rejection threshold while another copy is still in voting closes the same way. Their authors are told, and it costs them nothing: no reputation penalty, and it doesn't count toward the rejections that can lead to a contribution pause.
Two pending horses of the same breed are not always the same horse. A game can release several variants of one breed at once, and those are separate listings. Grouping is the staff decision that says "these are the same horse".