Will that get rid of the warning message?
Yes
Simplistically 'certificate' is just signed public key from generated pair of asymmetric keys.
Certificate must be signed with private key of some trusted authority named 'Certificate Authority'. CA may be public or private (yours). You may create your own CA with openssl, generate certificate and sign it by your own CA.
To check validity and trust of your certificate program or operating system needs certificate (public key) by what your certificate is signed - CA certificate.
To trust CA certificate itself it must be somehow marked as 'trusted' for program or OS.
Putting CA certificate into the 'Trusted Root Certification Authorities' store
literally means that you trust this CA cert and it become trusted by your OS.
Self-signed certificate is signed with its own private key, instead of CA's private key. So to check self-signed certificate
you have to trust the self-signed certificate itself (its public key) by putting it into 'Trusted Root Certification Authorities'.
Also is it possible to create identical self-signed certificates?
No
Prime numbers for keys (if RSA used) are randomly chosen from very large number space and the size of this space is proportional to the key's length (thousands of bits as for now, 2key_length) and chance to choose the same numbers is very low.
What are the chances to generate the same ssh key?