Correct spelling, explanation: opening is a gerund of the verb open. This verbal noun comes from Old English as openung. As we can notice, there is no extra consonant. It is because the rule says that we should add the suffix ing to the base and when this is a stressed syllable, we double the last consonant. But in the word open, o is stressed, not pen. Therefore, the correct form is opening with one letter n, whereas openning is incorrect.

Definition of opening:
noun/adjective, an act of making something not closed, accessible, or available for people,
“Hurry up. Where are you?” “I’m just opening the door.
She is always opening the window when it is cold. It is so irritating.
There is an opening in the roof in order to get rid of the smoke but they slipped through it to escape.

Phrases with opening:
open a can of worms – make a complicated, unpleasant, and difficult situation than you realized,
He opened a can of worms when he published his autobiography.
open something a crack – when you open a door or a window a crack, you do it very slightly,
He opened the door a crack to hear what she was saying.

Incorrect spelling, explanation: forming a gerund or a present participle from a verb, we add the suffix ing and when the verb ends in one consonant-vowel-consonant syllable or the last syllable is stressed, we double the last consonant. The word open originated from Old English, similarly to the gerund opening and ing ending although it was also known as ung ending and as a result, openung. It clearly shows that there is no additional consonant in this form. Thus, openning is spelled wrong, only opening is correct.

Incorrect spelling