Technically, it is Alioli in Spanish and Allioli in Catalan.
The name originates from the Catalan "all" for garlic and "oli" for oil, thus the double "l" in Catalan
In Spanish allioli drops an "l" and is spelt alioli as otherwise the pronounciation would not be the same as in Catalan.
The two forms are widely used in Spain so take your pick.
Just to make things a little more complicated garlic in French is "ail" and so on the other side of the border they write it aioli. Since many culinary names come from French, some people in Spain also call it aioli.
So you can't say there is no choice.