First usually if wanting to make a 20% product, the great majority of materials used would not have been prediluted to 10%.

Not that one may not be able to do it, but it's inconvenient.

If having to do it, determine the total weight of aromamaterials, determine the total amount of diluent (alcohol or other) needed, which is four times that, subtract the amount of diluent used already from your diluted aromamaterials, and add the remainder.

Simple example:

Formula is Hedione (undiluted) 200, Sampaquita (diluted for some reason to 10%) 600, Galaxolide (50%) 200.

Your total aromamaterials, not counting their diluents, add to 360.

Multiplying that by 4, you need 1440 total diluent.

You already have 540 from your diluted Sampaquita, and 100 from your Galaxolide, total of 640.

Therefore you need to add alcohol of 1440-640, which is 800.

And to check that calculation:

Diluent: 800 added at end, 540 in the Sampaquita, 100 in the Galaxolide, total 1440.

Aromamaterials: 200 Hedione, 60 from the Sampaquita, 100 from the Galaxolide, total 360.

Grand total, 1440 + 360 = 1800

360/1800 = 20%