'A double debt to pay' is a poetic reference popular in the 1800s, and entirely forgotten today. The relevant lines are:
"The chest contrived a double debt to pay, –
A bed by night, a chest of drawers by day"
--The Deserted Village by Oliver Goldsmith, 1770
In context, the meaning is clear: a double debt simply means that an object (or place, person, or institution) serves two purposes.