error: Compose file './docker-compose.yml' is invalid because: services.koha.networks.kohanet contains non-unique items

The cause is that the .env is not there (at the root of koha-testing-docker)

This is an issue just to document the fix and make it findable more easily. Due to the very confusing error message.

The full error is

ERROR: The Compose file './docker-compose.yml' is invalid because:
services.koha.networks.kohanet contains non-unique items, please remove duplicates from ['', '']