Here is a key to successful laminate flooring : use only material that is at least 10 mm, not including any underlay. Avoid 7 and 8 mm products entirely. 12 mm ( not including any attached underlay) is even better, but 10 mm not including underlay us minimum! It’s andy better to find narrower planks , less than 5” . Narrow thick planks are not nearly as prone to cupping, which occurs in cheap laminate. More on reddit.com

Solid Hardwood, Engineered and Laminate Flooring - Laminate w/ attached pad on concrete - which underlayment? - Putting 12mm laminate in the basement (I know, not the popular choice) and it comes with an attached padding. Initially, I was thinking of using the Roberts 3-in-1 underlayment because ... More on doityourself.com

The real issue is that the flooring is not made for that. If you add another layer of something that is semi-flexible underneath (not like a layer of plywood), then stepping on the planks can cause them to flex just a little bit more than intended. That, in turn, puts more pressure on the thin vinyl edge strips that interlock the planks, which can cause the planks to crack and separate. And then your floor is ruined. More on reddit.com

There's a difference between underlayment plywood (good) and underlayment padding (bad if already on your lvp). I think the contractor that wants to pull the existing laminate and carpet is saying to take everything off down to the subfloor, and add a new underlayment plywood (1/4" typically) on top to make it all smooth and the same height throughout. You're right that doubling up on padding is not a good idea. The new ply will give you the best base to start on, and the contractor recommending that sounds like the one that wants to do the job right. I'm not sure i understand the comment about raising the old carpeted area up "if necessary." It will absolutely be necessary if the adjoining laminate is left in place. Otherwise you'll have a 1/4" or 1/8" step down in the 12x12 area where the carpet was? Trying to match those elevation changes can be tricky. More on reddit.com