Science and biology are just a couple of areas when you shouldn't say things like it can't, never or won't happen.
http://www.time.com/time/health/arti...603799,00.html
http://www.baby2see.com/multiples/semi_identical.html
https://www.boundless.com/biology/fl...ion-in-plants/
http://multiples.about.com/od/funfac...niquetwins.htm
Can it happen yes, but it's not like anyone has their twin bps genetically tested to confirm it, even then it doesn't happen often but it still happens.