When user is publishing translation in CX, captcha can be shown to check if user isn't a robot. While user is typing captcha (once or more times, because of failed attempts), someone else can create page with the same name. We (will) warn users about already existing title (T190038) while they are editing and give "Publish anyway" option to potentially overwrite existing pages, but that is not covering this corner use case.
From gerrit comments on 427196:
There is a small race condition here: someone can create the page while the user is typing captcha. It is such a rare case that I think we can safely ignore it. action=edit has createonly/nocreate parameters which could be used to detect this case, but cxpublish doesn't have them.