That’s nonsense.
Who is paying inheritance tax twice on the same money?
Two different taxes being levied on the same money isn’t double taxation. If it was then paying VAT on anything you bought with money that had already been subject to income tax would be double taxation, or paying insurance premium tax on a policy you bought with money already subjected to income tax would be double taxation, or paying dividend tax or interest tax on money you had invested which had already been subjected to income tax when you earned the money would be double taxation.
These are different taxes being applied to separate transactions.
Inheritance tax is a bad tax for many reasons: it raises comparatively little, it creates perverse economic incentives, it is applied very unevenly with the largest estates paying next to nothing, there are good moral arguments against it also, and the method of implementation means that it causes a lot of unnecessary stress to inheritors in certain circumstances. However, it is not “double taxation”.