Correct spelling, explanation: the word opening is made from the verb open in which the first syllable is stressed. Hence, there is only one consonant n before the ing ending. The reason is the origin of the word and the rule of forming gerunds. The verb open has its roots in Old English and even its gerund or present participle as openung comes from this old language. In the form opening or openung there is no extra consonants n. It is because the rule says that we add ing ending and double consonant in the stressed syllable but pen is not stressed. Therefore, only opening is correct, the form openning does not exist, it is spelled wrong.

Definition of opening:
noun/adjective, when something is opening, it is ready to provide service; not fastened or not zipped; available for people to enter or participate in something,
It was raining and I tried opening my new umbrella.
Now I’m opening the Bible in order to find interesting thoughts.

Phrases with opening:
open something to someone – allow someone to use something,
My grandma opens her home to anyone who needs a place to stay.
open someone’s eyes – cause someone to notice something important,
After reading this book, they opened their eyes to the truth.

Incorrect spelling, explanation: openning is an incorrect form of gerund or present participle formed from a verb open because there are double consonants n. It may be a misspelling or a typo but it can also be spelled in this way because the users of language made a mistake, applying the rule of creating gerunds. It is true that we should double consonant but in the stressed syllable. However, in the word open, pen is not stressed and it is not required to write two consonants n. As a result, the correct form is opening, not openning.

Incorrect spelling