I do not often come across the word catsup, but I do see it every once in a while, and I know it means ketchup. What I don't know is why they both came to be words for the same thing (though ketchup is much more popular). Dictionary.com says catsup was invented later as an anglicization, but even that raises questions. Why and when did someone try to anglicize ketchup, and why didn't it ketch on?
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Etymonline entry is
So, there was catsup and catchup before ketchup and even the recipe had changed. Here's ngram for illustration of use
Wikipedia entry might prove to be an interesting read to you, too.