The first six double exposures were shot all with a double exposure setting but these two examples by Aaron Checkwood where shot at separate times. Meaning, he shot all the flower shots first, removed the film, then reshot all the wave shots, overlapping them. When doing this technique you need to use the same camera and pay close attention to putting the film in the same way so your frame lines match. To see more of Aarons work check out http://www.thecheckrepublic.com and follow him on Instagram at @the_check_republic