The duty of a clerk is to check all incoming mail and to record it.
The sentence above should be revised into
A.
It is the duty of a clerk is to record all incoming mail and to check it.
B.
It is the clerk's duty to record all incoming mail and to check it.
C.
The duty of a clerk is to record all incoming mail and to check it.
D.
A clerk checks and records all incoming mail.