This "multiple taxation" nonsense is nonsense.
Everything is taxed multiple times: I pay income tax on my salary. I spend my salary buying apples, and I pay sales tax on this. From the money I paid for apples, the store pays the cashier's salary, and part of this goes to income tax. The cashier spends their salary on software from my employer, and part of this goes to the state as sales tax. My employer uses the cashier's money to pay my salary, and I pay income tax
again to close the circle.
Money flows in circles, and every single transaction in the chain is subject to taxation. Picking out one particular tax on one particular kind of transaction -- surprisingly, one that only affects rich people -- and claiming that it is somehow extraordinary is intellectually dishonest. Abolishing the inheritance tax is what would create an exception from the general rule.
As for:
> why is an inheritance tax needed?
As a method of redistribution from the very rich to the very poor, to create a more level playing field for everyone. (I'm not claiming that there is a particularly level playing field anywhere, especially not in the US. But abolishing inheritance taxes would certainly not make it more level.)