This happens frequently when the person or historic event honored has a special meaning for both countries. Franklin had been the first Postmaster General of both Canada and the United States. Both stamps were designed by the same person and looked nearly identical. An overprint is any printing added to the face of a stamp after it has been manufactured. This is usually done to make stamps suitable for use not originally intended.
Precanceled stamps represent one kind of overprinting.
Whenever an overprint changes the value, it is called a surcharge. A watermark is a special design or pattern pressed in to the soft paper during the manufacturing process. These impressions take many forms, such as letters, animals, fruit, emblems, and combinations of these and others. They are frequently hard to find under normal inspection. Watermarks help provide protection against forgery.