[General] One should do his duty honestly

[faisaljan]

Is it grammatically correct?
One should do his duty honestly.

 

[Tdol]

No- if you use one, use it consistently:

One should do one's duty honestly.

 

[Raymott]

Is it grammatically correct?
One should do his duty honestly.

No. "One should do one's duty honestly", "He should do his duty honestly", "I should do my duty honestly", etc.
That is, unless you're talking about doing another person's duty honestly.