The word “redouble canvas” is usually used to reinforce a very old and “brittle” painter’s linen. The cloth is aging “and crumbles” by age. Sometimes it happens in combination with the repair of a big tear. The restaurators call it also “doubling” (doubling the cloth). The painting is taken from the frame and glued with reversible glue on a new piece, 1st grade, painter’s linen and then re-stretched on the old / existing frame.