Had had

PreciousJonesWhat's the difference and which one is correct?

There's no significant difference between them and both are accepted.
Normally, to refer to something happened in the past, we use past tense when we know the (exact) time of the action, and past perfect tense when we don't know.
I think.

Had is used for something that happened in the past. Had had is used for something that had already happened at a particular point in the past. In your example I prefer had, because "as a child" is a period of time during which he had polio, not a point in time before which he had polio.
She had chicken pox when she was seven, measles when she was eight, and mumps w

Blue JayJohn, where Fred had had "had" had had "had had". "Had had" had been correct.

I can

John and Fred had to use either had or had had in a particular sentence, like the one Precious Jones asked about. Fred incorrectly used had. John used had had, which was correct.
I was told this one by a Frenchman, who had to explain it to me using the French equivalents of had and had had before I understood it myself.

Blue JayJohn, where Fred had had "had" had had "had had". "Had had" had been correct.

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