My friend and I have been arguing about this for a long time. He claims that it is correct, but I believe it to be incorrect.

In Ontario, everybody speaks like this.

Instead of saying "who is coming?" or "what is in your collection?"people say:

"Who all is coming?" "what all is in your collection?"

To me, it just sounds completely incorrect, and it drives me nuts.

I'm wondering if it's actually correct, or just slang/incorrect.

ThanksEmotion: big smile
No its not correct. It sounds like local slang to me.
Anonymouspeople say:
"Who all is coming?" "What all is in your collection?"
Yes, they do. Sometimes I do, too. I can't believe that I would say anything incorrect, so it must be correct. Emotion: big smile

I'll be interested to see what different comments you get on this one!

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In Ontario, everybody speaks like this.

I live in Ontario and I don't speak like this. (not usually.Emotion: smile)

As noted, it's casual and slang-y English.

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I'm with Jim on this one -- it doesn't sound wrong to me. "Who all is coming?" is asking for more information than "Who is coming?" I can't think of a better way to ask "who is coming" and indicate that you want a complete list.
"Who all is coming" sounds extremely weird to me. I've never heard anybody saying that, so I'll say it's wrong.