Theory:

     Now Bepin Babu realised that it was not he but this man who was making a mistake. Bepin Babu had never been to Ranchi. He had been at the point of going several times, but never made it. He smiled and said, “Do you know who I am?”
 
     The man raised his eyebrows, bit his tongue and said, “Do I know you? Who doesn’t know Bepin Choudhury?”
 
     Bepin Babu now turned towards the bookshelves and said, “Still you’re making a mistake. One often does. I’ve never been to Ranchi.”
 
     The man now laughed aloud.
Explanation:
 
After listening to Parimal Ghose talk for a while, Bepin Babu realised that it was no longer Bepin's mistake, but Parimal had mistook him for someone. So, Bepin concluded Parimal Ghose had misidentified him. Bepin informed Parimal Ghose the same that he was confusing Bepin with someone. The reason was that Bepin had never been to Ranchi in his lifetime. And so, there could be no opportunity for Parimal Ghose to be familiar with Bepin.
 
Though it was a fact that Bepin had intended to visit Ranchi on several occasions, he never went there. So, Bepin felt more confident in informing Parimal that he was not the person he was referring to. Also, as Bepin had more confidence in his memory, he asked Parimal, "Do you know who I am?" to which Mr. Parimal Ghose replied more confidently that he was Bepin, whom he knows very well.
 
Bepin was surprised to hear such an answer from Parimal, and he disagreed with Parimal's claim. As Bepin had never been to Ranchi in his lifetime, he informed the latter again that he was making the mistake of confusing Bepin with someone else. Bepin then turned toward the bookshelves, as he didn't consider Parimal's speech to be serious.
 
He again told Parimal that he had mistaken Bepin for someone else. Also, he convinced Parimal by stating that such an identification mistake may happen to anyone. He also revealed to Parimal that he had never visited Ranchi previously.
 
Mr. Parimal Ghose was unconvinced by Bepin's response and began to laugh at him for not remembering his Ranchi trip.
 
Meaning of difficult words:
 
 S.No
Words 
Meaning 
 1.
Familiar Someone or something well known to you; Often seen or heard before 
 2.
DisagreeTo not to agree
 3.
RevealTo make something known to someone that was unknown before
Reference:
National Council of Educational Research and Training (2008). Honeydew. Bepin Choudhury’s Lapse of Memory- Satyajit Ray (pp. 59-69). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.