index problem with List of GameObject

Greetings,

I have this script where I spawn various prefabs each update if certain conditions are given. The script chooses a random prefab from a list, that is either in it's personal list (for the first one) or the list of the previous spawned prefab (if there is one).

Some prefabs have themself in their personal list. Wich is causing an exception as soon as the script wants to spawn that prefab. Because there is none, for some reason. Instead of saving the prefab in its list, the prefab saves its own instance in the list.

So I'm trying to compare a GameObject instance, with a prefab (but that doesn't work):

 int pre, fab;
 if (previousPfc == null) fab = Random.Range(0, availablePrefabs.Count);
 else {
    pre = Random.Range(0, previousPfc.allowedFollowups.Length);
    fab = availablePrefabs.FindIndex(x => x == previousPfc.allowedFollowups[pre]);
 }

Does anyone know a workaround for this?