Said here is used in the same way that "the, this, that chair" is used. It is used to identify which chair is being discussed.
I could say, "Pick up that chair" or "Pick up the chair" or if somebody had been talking about a chair that, let's say, had a special recline lever and I wished now to refer to it I could say, "Yes, but does said chair [the one you were just saying something about] have a built-in massage function?" I could have as easily said "your chair" or "that chair".
It is often the case that said is used when the object of said is of dubious existence. So if somebody says, "I know of a place where money grows on trees." I might ask, "Oh, really? How do you get to said place?" By which I mean, I agree you've said it exists, but I don't really believe you.
Having said all that, using "the said" together is at least not idiomatic if not ungrammatical.