There are loads of sources for it, it isn't as odd as you would think, it is even listed on wikipedia with lots of nice sources. Apparently "their number is thought to be increasing." and "In one case, a double defector re-entered North Korea four times." [0]

Edit: I added the link to the nutrition study below also. [1]

[0] https://en.wikipedia.org/wiki/North_Korean_defectors#Double_...

[1] https://synapse.koreamed.org/search.php?where=aview&id=10.41...