Why is "if" used here? Doesn't it have to be "that"?

Question

There was a rush to buy the cook drinks and hear more details. 

"Always thought he was odd," she told the eagerly listening villagers, after her fourth sherry. "Unfriendly, like. I'm sure if I've offered him a cuppa once, I've offered it a hundred times. Never wanted to mix, he didn't." 

Harry potter and the goblet of fire

Instead of if, doesn't it have to be that or something? She's sure that she has offered him a cuppa, quite a lot of times, actually, but he never wanted to join her.

Or if here does mean if, does this sentence mean "if she offered him a cuppa (though she never did), he would have certainly refused it no matter how many times she tries"?

I don't understand this sentence.

Answer 1

You are right that there should really be a that there, but it is often omitted in informal spoken English, and this is very informal. You can read more about when it is permissible to omit that here.

The following sentence is a colloquial phrasing that is intended to emphasise that you have done something a lot of times. Here is the template for it:

If I've told you once, I've told you a thousand times.

You can read more about this expression here and here.

Adding "I'm sure that..." at the beginning adds further emphasis.

You are right that it means that she did offer him a cuppa a large number of times, and he always refused.

Answer 2

I think this sentence could be expanded to say "I'm sure that if I've offerend him a cuppa once, I've offered it a hundred times."

It is similar to saying, "If you've seen one high school, you've seen them all" (meaning they are all the same).

In this case (I haven't read the book and don't know the contxt) I presume this is a waitress talking about a patron. Waitresses often come up to clients and offer drinks sort of automatically, without thinking about it (a lof of things that we do often become automatic for us). What she might be saying is that she doesn't remember for a fact that she offered him drinks many times before, but if she remembers this one time when she offered it, it also means she had done that many times before.