You may have examples of 'instructional markings', which were intended to be struck on cover and not the stamps.
PD : Paid to Destination. Another variation can be PP (Porte Paye) : Postage Paid. These can found in circular/rectangular/rounded square frames; and not always in red ink.
I'm sure there are others have far better knowledge of these than I.
As far as the original question goes, these PP/PD marks were used on foreign mail to show that it was appropriately prepaid. They were struck anywhere convenient on the cover so it's not uncommon for them to have overlapped a stamp by chance.
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