Cannot summon multiples of the same creature
In vanilla you can summon multiple instances of the same creature by creating a new spell, so that Summon Skeletal Minion and Summon Skeletal Minion II each summons a skeleton. In OpenMW both spells fire, but the second spell either doesn't summon anything or overwrites the first spell.
(RM-2194 from redmine: created on 2014-12-08 by Carl Daugherty, , closed on 2015-01-07 by scrawl ., relates #2135 (closed))