prisma/web#4188 raised a valid point that it is bad to use the same database for shadowDatabaseUrl as you are using for url - your database will be overwritten and you will lose data. The PR adds a warning into the docs, but we can actually do one better and just fail validation of the schema in this case.

Internal discussion: https://prisma-company.slack.com/archives/CGMCB82N8/p1670232135999259