Ensure that your Letter specification has a unique Title. It may be that you have more than one Letter spec with the same Title and are selecting a different spec than what you mean to. Use the Designer to open your 1-page spec and from the File / Specification Options, not only ensure that the # of copies is 1, but also edit the title to something unique. Then save and go back to the Letter printing function and try again.

Letters can only have 1 section, say a Page Header. It cannot have multiple page headers or footers, and definitely no detail sections.

Does a sample Letter spec do the same thing with 3 pages? If not, then there is something in your Letter spec that is not defined correctly.