One clear difference is that helpful can be used to describe a person's attitude: "She is always friendly and helpful ", whereas useful is not generally used in that way. Something like a tool or a piece of information would be described as helpful if it helps you to do something (especially if it makes it easier to do something that you could already do without it), and useful if you can use it in the process of doing something (especially if it can be used in many different situations). There is obviously a lot of similarity here, but still a distinction: " useful tool" vs.
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