If you are willing to pay 20 Rs and your friend is willing to pay 10 Rs for the same commodity, even when income in hand is same for you and your friend.
Why is your friend willing to pay less for the same commodity?

marginal utility expected by your friend is worth 10 Rs and marginal utility expected by you is worth 20 Rs

Price that the consumer is willing to pay for a unit of a commodity is always equal to marginal utility of the commodity. Marginal utility refers to the additional satisfaction derived by consuming 1 extra unit of the commodity. So, any rational consumer would at least expect the same amount of utility as of the price he is paying. For me the good is giving satisfaction worth Rs. 20 but for my friend the satisfaction is worth Rs. 10 only. Utility is a subjective concept which varies from person the person.