Effects in constant spells should not be merged
Based on this topic.
Our current implementation of
MagicEffects collection uses spell effect ID as a key. If there are several effects with the same ID in the same source, it merges them internally and sums magnitude.
It is usually OK, but this approach has a drawback - it is not possible to summon multiple creatures with the same ID via constant spell since OpenMW merges these effects to the single one.
It is not too important, though, and I do not aware of any mods which actually use this feature. For constant enchantments this issue does not exist.