The money changers in Arcade, Raffles place are again offering a rate better than the spot rate. The current spot is 47.20 and you could get 47.75 with the money changers.
Again the notes are all legit and there is nothing wrong that I could find.
Last time this happened, it was in October 2016 and the 1000 and 500 Rupee Notes were Demonetised in on 8th November 2017 (https://adityaladia.com/2016/10/11/cash-rate-of-inr-better-than-spot-in-arcade/)
I could not help but wonder if 2000 Rupee notes will soon be withdrawn and prove the rumours correct. If that happens where will Indian GDP go is anybody’s guess but till then if you are visiting to India then exchanging money in Singapore and carrying back is a profitable bet.