Dual roles did not contribute to Mohanlal's success. This is evident from his movies like Mayamayooram and Udayon. Both were not commercially hits. The exception in this list is his hit movie Ravanaprabhu. Still, he has accepted a dual role for his forthcoming movie Photographer.
Photographer is directed by Ranjan Pramod, his first movie in Malayalam. In Photographer, Mohanlal is playing a role of a photographer (what else?) of wildlife; the other role is that of a farmer.
The crew is zipped from revealing the character of the farmer. This is been done to avoid the jinxed luck of his earlier dual roles. The director is under a gag order not even to publish news or expose the character of the farmer. Anyway, best wishes for the success of the movie.