I have met others who are allergic to MSG.

I think the allergy does usually hit badly the second 

exposure ( For a logical but complicated reason that I can't remember ).

My allergy is Oysters. Not the first time, just the second time onwards. In France they eat them at Christmas ( bizarrely ) and while staying with my brother in France, one xmas , his friendly neighbour offered me one. I thought the allergy may have gone away, or been a figment of my imagination ...and anyway ONE wouldn't do me any harm. So I ate one and it took just a few minutes to provoke a violent and unpleasant reaction....in my case just explosive vomit. Won't eat them again.

Best wishes to all after that delightful story