IHTM14730 - Lifetime transfers: specific lifetime reliefs: disallowed debts: transfers chargeable when made
Where you are considering additional charges (IHTM14571), you can allow double charges relief as you would for a potentially exempt transfer (IHTM14701), and follow the same calculations.
As the calculations are of tax payable as a result of the death, the lifetime tax paid at half-death rates is not affected.