unsound mind challenge on registered will?

Normally a registration authority would not agrees  to register a document  without  satisfaction .   Need more clarity on your query

Collect / submit evidence(s) before concerned (probating) court to the effect that the testator was of unsound mind at the relevant time (when the will was executed).

The very purpose of Registering the Will is to ensure that everything is as per law. Registrar doesn't permit Registration if he has any doubt as to the mental status or independent decision without coersion.

Registration notwithstanding if the Will is to be contested, it requires a very strong evidence based on real facts and documentary evidence, which you should be able to establish beyond doubt that were suppressed at the time of Registration.

Even a Registered Will has to be probated in High Court of judicature under such conditions of property situated in metropolitan cities etc.,

You have not posted the material facts.

Was there any doctor, as an attesting witness to the WILL?

Since you are challenging the will, the burden of proof lies on you, to prove the fact that the will was executed during unsoundness of mind by the Testator/Testaterex. All kinds of unsoundness of mind unconnected with the testamentary disposition of property would not deprive the Testator/Testatrex the power to dispose of his/her property.