As I said, go and educate yourself. Show is irregular aswell as it differs in past participle.
Showed is NO synonym for shown, that is no inofficial language that is just wrong.
Depends how you look on it, whole USA, Canada, Australia etc. use a wrong form?
It's called informal language, it's a mistake according to Oxford Dictionary but people use it in a colloquial speech. You can notice it in every language all over the world.
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