Pergo brand flooring with the attached underlayment. When installing should I use any additional underlayment?
Go with what the brand recommends or the flooring instructions specify.
Compression of additional underlayment, under normal use dynamic loads (walking etc..), may cause unnecessary movement and flexing at the seams / joints and thus possibly lead to premature failure.
This can result failure of the moisture seal at the seams, or mechanical locking, or cracking of the tile.
For planks with attached underlayment, it is not acceptable to install additional underlayment prior to flooring installation.
The instructions will be painfully clear about this. You really should read them thoroughly to achieve a satisfactory outcome and not void your warranty.
I did that mistake once. Thinking it will reduce the noise and provide better thermal insulation.
Well it did not, it made it worse due to increased flexing of the boards under load.