If estate duty has already been paid in respect of any settled property since the date of the settlement, on the death of one of the parties to a marriage the estate duty shall not be death of one of the parties to a marriage the estate duty shall not be payable in respect thereof on the death of the other party to the marriage, unless the latter was at the time of his death, or had been at any time during the continuance of the settlement, competent to dispose of such property, and, if on his death subsequent limitations under the settlement take effect in respect of such property, was sui juris at the time of his death, or had been sui juris at any time while so competent to dispose of the property.