I think that the maximal renewal time should be proportional to the expiration time.
If, for example, the original expiration time is 5 minute, during the first 5 minutes any new resubmission of the URL gets the same address, and the time is extended to 5 minutes. After these 5 minutes, the address is still available if it life was extended, but any new resubmission get another address.
In this way the shortcut is active between 5 and 10 minutes, and at any time there are at most two shortcuts for each original URL (for each expiration interval).
If, for example, the original expiration time is 5 minute, during the first 5 minutes any new resubmission of the URL gets the same address, and the time is extended to 5 minutes. After these 5 minutes, the address is still available if it life was extended, but any new resubmission get another address.
In this way the shortcut is active between 5 and 10 minutes, and at any time there are at most two shortcuts for each original URL (for each expiration interval).