For various reasons some "private keys" are public.
Example private keys are part of standards, software test suites and public documentation. Sometimes such example keys are used in production due to a lack of understanding how public key cryptography works.
Firmware often comes with hardcoded default keys. In the past security researchers have published collections of such hardcoded keys, e.g. the littleblackbox and the House of Keys.
No matter why a private key is public, it is obvious that a "public private key" should be considered as insecure and compromised.
The "Kompromat" repository collects "private keys that have become public". We use this as the basis of our blocklist.