How common is this? If only we knew. Political correctness has squelched probes of noncitizen voting, so most cases are discovered accidentally instead of through a systematic review of election records. The bottom line is that the honor system doesn’t work. There are people—like those caught voting illegally—who are willing to exploit these weaknesses that damage election integrity.
https://www.google.com/search?q=Do+Illegal+Votes+Decide+Elections%3F&oq=Do+Illegal+Votes+Decide+Elections%3F&aqs=chrome..69i57j69i60.62j0j7&sourceid=chrome&ie=UTF-8#q=Do+Illegal+Voters+Decide+Elections%3F&tbm=nws