IMAP doesn't actually have folders, they're an artifact. (Each email is kind of "labeled" as belonging in a certain folder, so the app represents it that way.) Some apps are better at it than others. If the email provider has a webmail page (gmail does - GMail), go there and delete the extra "folders". (Be careful - look at what mail is in each - move what you want to keep to the folder you want to keep. If it's the same mail in each one, don't delete anything, you're just seeing the same folder twice - deleting one could delete both.)