[General] The doctor asked me

The doctor asked me how long I had had the symptoms for.

Why do we two "had"? Can we use "have had" instead?

The past perfect 'had had' is used because it refers to a period before 'The doctor asked me'.

Direct speech:
[doctor says]: How long have you had the symptoms for?
present perfect.

Indirect speech (need to backshift):
[I say]: The doctor asked me how long I had had the symptoms for.
past perfect.

http://learnenglish.britishcouncil.org/en/comment/86737#comment-86737
I don't know whether it is correct, but it says 'had been having' is more likely as it suggests the patient was still suffering at the time of his conversation with the doctor.

Matthew Wai said:

The past perfect 'had had' is used because it refers to a period before 'The doctor asked me'.

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The doctor asked me how long I had the symptoms for.

Does this sentence make any sense?

I think the direct speech of it is 'How long do you have the symptoms for?', which does not seem correct to me.