LAN cabling in single-family house (prefabricated house)

Duplex cable is usually just a convenience for the electrician... it doesn't really have any added value.... two single cables are usually cheaper than the same amount of duplex.

two individual cables are usually cheaper than the same amount of duplex

I have actually noticed the opposite. However, the difference is negligible anyway. We're talking about 20..30 euros of material for an average installation.

...you can bend the cables however you want, as long as you comply with the manufacturer's installation guidelines...the reason is...as described in DIN 18015...data lines can be laid interchangeably.

That's true, the electricians have their DIN. But LAN or coax cables can only handle squeezing and bending to a limited extent.

And for most electricians, a cable is considered finished once all the wires can basically be measured through (in terms of resistance).

With 1/10 Gbit cabling, we have already found several cables that simply didn't work properly.

Cat7 has 8 wires to be able to fully utilize it on 1, so is it better to have one cable per socket, and with two sockets two cables, right?

hbf12 has already explained it regarding the aspects of Gigabit Ethernet and PoE. The second reason is that today it is generally recommended to wire "structured." That means cables and sockets are designed to be usage-neutral. For this, all eight wires are needed as Ethernet uses 1/2 and 3/6 (Gigabit Ethernet even uses all pairs), ISDN uses 4/5 and 3/6, and analog telephones / fax machines only 4/5. If all wires are connected to the sockets, the usage can be changed without having to "catch up" on wires that were not previously connected. So much for the reasons for "all eight wires."

Furthermore, nowadays IT cabling is also done in a "star topology" — not as a "bus" like the S0 in ISDN — and therefore each socket is connected individually. A double socket in this sense is electrically two sockets, even if they share one socket housing. That is the reason for two cables to a double socket. Whether the cable pair lies loose side by side or glued together is a "matter of taste."