They do seem to happen more than you would expect but I think still rare and unpredictable.

Per the theory that they are chimeras they could then only occur in clutches that would have mixed morph types. For example a clutch that could produce both albinos and non albinos (hets or normals). If both parents where albinos and two of the eggs merged to make a chimera it would still be all albino looking and not be recognized. You need different morph offspring to merge to see the paradox effect.