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]
I was questioning your claim of "surprisingly large" double defectors not that there are defectors. 13 out of ~1600 is not a significant number. Your second source is not relevant.
>"In one case, a double defector re-entered North Korea four times."
Constantly sneaking in back forth is a common thing from the documentaries I've watched. 1. People stealing goods and selling them back home 2. People stealing supplies to feed their family back home .
Thirteen publicly acknowledged by South Korea, and 700 missing. Most of which, if you read the cited article are thought to have defected back or attempted to.
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...