How do you keep network documentation accurate without tracking everything twice?

Skip the individual dynamic DHCP leases. They change constantly and the DHCP server already knows who has what. Document the scopes, reservations and static infrastructure addresses instead, along with VLANs and subnets.

Same idea for DNS. Record how the zones and forwarding are set up, but use exports or the DNS system itself when you need the current records rather than maintaining a second copy by hand.

NetBox or phpIPAM is worth a look for subnets and address assignments. NetBox can also hold device and connection information, so you can keep more of the inventory in one place instead of spreading it across diagrams and spreadsheets.

Pick which system is authoritative for each kind of information and make updating it part of a change. If you use configuration management, you can also consider feeding values from the inventory into it. The tool won't prevent drift on its own.

Use discovery to check the inventory, not as proof that every entry is correct. Compare DHCP leases and reservations, DNS records, switch port information and an authorized scan such as Nmap service detection. When they disagree, investigate before changing the documentation. A scan is a snapshot and won't necessarily see every device or service.

Keep the diagrams useful rather than exhaustive. A physical view of racks and major switch links plus a logical view of VLANs, routing, security and site connections can make troubleshooting much easier. Add detail where it helps, such as cluster connections or important virtual machines.

The big thing is tying diagram updates to the change process. A perfect map that nobody has time to maintain will become a guessing game again.

A shared wiki is good for the stuff an inventory can't explain. Put network design rules, configuration templates, operational notes and the reasoning behind unusual settings there. Update it when something changes or when troubleshooting uncovers a gap. Even a small correction is better than waiting for a complete rewrite.