Question

why is double co incidence of wants necessary in barter system​

Answer 1

This is because if one party has a shortage of one thing but the other thing in excess.while the other party has opposite case means having more quantity of the thing which was less in party one .amd is required with the thing that is in abundance in party one.
means both the parties are deficient in goods which other party have in excess .so they barter these goods and yes we cn see it as coincidence of having shortage with the thing other people have in large amount.
hope it helps you

Answer 2

Double coincidence of wants is necessary in barter system because in barter system people exchange their commodities without the use of money. It requires that both the people agree to buy each other's commodities. But when money came into existence it is not necessary that both the people have to buy other items.