In wheel patterns exposed with a relatively high dose, a lower dose second exposure can be seen that is shifted to the lower right.

Images of a gold standard taken at different kV's and using different apertures also showed the double image, which indicates that the problem was from the field emission tip. Decreasing the emission current of the tip significantly reduced the effect. Apparently, a defect on the field emission tip was emitting a secondary beam, thus causing the double images and double exposures.