You can laready do multiple sumons by having multiple instances of a spell. Just create your own version of the same summon and voilla! You have multiple.
Or you could just put multiple summons in the one spell (but that makes it really tough to cast)
No, you cannot create a spell at a spellmaker that has duplicate effects, other than fortify attribute and fortify skill effects effects. That is probably because there are different attributes and skills to fortify.
You can, for example, make:
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
Fortify Alchemy 100 pts on Self
To make super potions. But you cannot make:
Fire Damage 20 pts in 5 ft on Touch
Fire Damage 10 pts in 10 ft on Touch
Fire Damage 1-5 pts in 10 ft for 3 secs on Touch
Or:
Summon Skeletal Minion for 60 secs on Self
Summon Skeletal Minion for 60 secs on Self
If you try to add a duplicate effect other than fortify attribute/skill, you get a message stating: "This effect has already been added."
While you can make such spells in the CS an then add them to a spell merchant or through the console, you cannot make such a spell in game; which is what I am asking for. As it is doable with fortify attribute/skill effects, it may be possible with the others.