What problem does this solve or what need does it fill?

Currently when two cameras exist on the same render layer, without explicit ordering, warnings will be printed to the console on every frame. While important to let users know about, this does make it hard for users to see their own logs, including logs that they may be using to debug the issue.

What solution would you like?

I'm not certain warn_once would work as it might confuse users into thinking that issue has been solved. Perhaps logging once per second?

What alternative(s) have you considered?

Leave it as is. This is already an uncommon issue and the only real case where the solution isn't easy is when importing scenes that have cameras which will be solved when the editor is released.