If you know the sequence, order primers from both ends and then perform PCR, one should get dsDNA.

The fragments are having different sizes (6-20kb for now) and I want to make them as long as possible double stranded.

Dear Thuur 

Use random hex in a Klenow reaction to make dsDNA from ssDNA.

SD

The answer is same as Dr Sibnarayan Datts',

You could try to use a RACE kit aproach, this will ligate adaptors to single stranded DNA ( for instance with terminal transferase and dCTP.) You could then use a poly-G primer to synthesize the second strand.

Whole genome amplification? Similar to the Klenow, but then with phi29 polymerase instead of Klenow. But do not know if this works (it also makes use of random hexamer primers).

If you have more fragments, different sizes you should synthesized primers for every of them and do the PCR or RACE is a very good solutionor maybe Genome Walking procedure...

Thanks all, I think we should use hexamer primers with Phi29 as polymerase. 

Hi Thuur, I was looking for the same answer for my research. Have you tried Phi29 for the second-strand synthesis?

Have you tried one protocol, please?