Both spellings may be found.
Who is the authority that decides in these questions?
***Neither a teacher nor a native speaker.***
A lot of things may be found, but you should not pay attention to everything you read.Both spellings may be found.
The general rule is to add an -ly on an adjective when you want to make an adverb out of it.
Of course there are exceptions, but when it ends on an l, you can simply add an -ly.
Normal -> Normally
Usual -> Usually
Grammatical -> Grammatically
Logical -> Logically
As said, don't take the rule too seriously, but these 4 examples should show you how to handle such words.
Thank you Shaisahab for your answer.
Dankeschoen Nightmare because besides giving me a fish you also taught me to fish.