My 4 year old son has also recently been diagnosed with DMD. His first tests came back with a duplication of exon 19. However because only one exon was affected they have re-tested him because apparently when only one exon is affected it could be an artefact. I am still waiting for these results but am expecting the duplication diagnosis. As far as supplements go I don't think the mutation type determines what supplements you take. For now I have been prescribed VitD. I also want to look into other supplements.
Dr. Kevin Flanigan, principal investigator at the Center for Gene Therapy at Nationwide Children’s Hospital in Columbus is as far as I am aware the only one that has started on developing a therapy for duplication mutations using the same exon skipping approach that is currently being developed for deletions. Unfortunately this work is not nearly advanced as it's deletion counterpart.
Here are some links. Hope they help some.
http://www.cureduchenne.org/press/new-research-points-to-route.html (Lynne: brief press release)
http://cureduchenne.com/blog/duchenne-duplication-and-rare-mutations/ (Lynne: a webinar where he discusses his work on duplications)
http://www.nature.com/mtna/journal/v3/n3/full/mtna20148a.html (Lynne: excellent deep scientific detail)