I've finally unlocked two ingredients for nigiri sushi. To maximize stats and value, should I put two of the same ingredients or different ones on there assuming I have all the ingredients and toppings unlocked.
You should definitely use different ingredients to create a dish. The reason is that the bonus from a good ingredient/topping combination is only applied once, even if the ingredient is present two times. So for the highest bonuses you should use two different ingredients with toppings that are good combinations for both, giving you a bonus of up to four A.
An exception is when you want to create a dish with a high tastiness but low cost, for example to level up costumers that don't have much money like the bookworm. Double Kalbi or similar works well there.