Duration of the increased fire damage effect does not overlap. Subsequent castings will not increase the duration, unless the second casting is performed with the Goetia Chausses +2, which will increase the duration to 90 seconds from initial casting.
Each successive casting of Firaja increases the damage by an additional 5%. It is currently unknown if there is a cap to the bonus damage percentage.
Casting a different -ja spell will remove this effect and initiate the timer for the new spell's element.