What do you mean by "confirm the validity" ?
Your employer is accusing you of producing false certificates?
And by extension, accusing the doctor of doing so?
Big call.
As to clerical staff getting involved - it'd probably be OK for a receptionist or clerk or somesuch
to say something like "Yes, that doctor works here" and/or "Yes, that's our standard format".
Basically, "yes, it's real", but that's all.
As a general thing (workers comp excepted) the doctor is not required to
disclose the nature of an illness to a patient's employer, just because the employer asks.
After all, it's not as if the employer knows better than the doctor.
As to clerical staff revealing clinical content, certainly not.
No responsible doctor would permit their layman employees to do that.