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The problem gets even more complicated when there are additional duplicate pages that can be reached from both domains.
Since the search engines aren't perfect, one solution is to use a 301 redirect to redirect domain.com to otherdomain.com/guide. That assures that only one copy of your content will be in the search engines at any one time, and should eliminate a possible duplicate content penalty. If you don't do this, chances are you won't get a penalty - it just depends on how cautious you want to be.
Your visitors can still type in www.domain.com, and get to where they want to go. This type of situation is ideal when you are trying to do non-search engine marketing, say through email or other advertising, where it's easier to remember domain.com as opposed to otherdomain.com/guide.