Limit entity spawns

I am making an entity that spawns in deserts, but I am having difficulty with the spawns. I only want one of them to be spawned in at a time, and I can't find a way to despawn others when the original is spawned in. I tried running a procedure that checks a 100 block radius around it for the same entity, but the issue was I believe it counts the original entity, so when I checked it with spawn eggs, it despawned the first entity. Any ideas?