YSK it's either "should have" or "should've". "Should of" is incorrect
Why YSK?
The first person who typed "should of" probably heard of it in real life that was meant to be "should've", they typed "should of" online and readers thought that it's grammatically correct to say "should of" which is in fact wrong and it became widespread throughout the years on Reddit.
I hope something could start to change.
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It may seem normal, because so many people do this. It is still incorrect, however.
That's not how linguistics works though. If people (native speakers) speak like that, it's "correct" or normal for their dialect. This doesn't mean it's "correct" in whatever is considered the "standard" dialect of the language (for English, there isn't one single standard, but de facto there are standard dialects in the English speaking countries which are taught in school and typically used in the news, newspapers etc.). But from a linguistic perspective, both "I have seen it." and "I seen it." are equally "correct" (linguists typically don't use that term in this context, rather something like "grammatical"), they just represent different dialects of English.