I found this strange problem in several articles recently translated with CX2 : ISBN are added twice to the translation, the first one is correctly formatted with the {{ISBN}} template, the other is using [[Special:BookSources]] which should be avoided for readability.
Exampe on frwiki with Anarchisme explorateur, but I saw the same problem on other articles by different users (HMS Avon Vale (L06), Toxophilus...) and only recently (at most a few weeks).
{{ISBN|987-580-092-9}} [[International Standard Book Number|ISBN]] [[Special:BookSources/987-580-092-9|987-580-092-9]]
Related: T213255: CX2: Doesn't handle correctly ISBN, should not put nowiki tags around them
I created This page compiles examples of the issue and a a quick test link to try translating them.
As shown in the screenshot, the issue seems to be on how the ISBN elements are rendered in the source, where some instances are already decomposed in several elements (template + link) that get transferred into the translation. The most surprising part is that the issue happens on some instances (see 2nd bullet point) but not others (1st bullet point), being all of them using the same template. So maybe these being part of a list or processing them in sequence is making a difference.