95 730il E38 rear window replacement ? double glazed or single ?

I don't think you have a choice in the matter.

There are laws that define the glass that must be used, and it requires two layers with plastic goo in between them to hold the glass together if it breaks.

That's laminated glass - different from double glazed.

Double glazed has two sheets of possibly laminated glass with an air gap between them - to reduce noise transmission.

I know what dual-glazed is, I am not aware of it being available in an automobile because of the curved surface of auto glass. I was thinking the OP used the wrong term. Dual glazing in a home is an effort of climate control -- heat transmission is reduced in a dual glazed window.

Are you saying that they make dual glazed windows for cars? If there is such a thing, I would guess that the design considerations necessary to support dual glazing would make conversion back to a single glazed window cost prohibitive, which should make the decision on which to use pretty easy.

It is.

Used for noise reasons on cars - as I said.

Yes.

The OP reckons it was an optional extra on his car. So it is possible a standard screen can be fitted as I doubt the body itself is different.

Double glazed or dual glazed windows are an option on some BMW and Merc cars that on the E65 in the UK is about £2k and it covers both rear doors and rear screen it is also heavily tinted or sun protection film applied.

I did not know that.