Why is it considered a good technique to double spot and ensure that the spot is not too big before developing the paper chromatogram?

It is a good technique to double spot because the spot of the material that is to be used for the chromatography process contain both known and unknown materials which have different concentrations. Doubling the spot, will allow those materials with little concentrations to have enough quantity to run through the chromatography process. Allowing the first spot to air dry before applying the second spot on the top of the first one ensures that one increases the concentration of the spot without increasing the spot size.