First, the document map feature is flawed and may mess up your document. This is not true of the Navigation Pane in Word 2010 and later. Word 2003 also has a flawed Document Map, but does not show the anomaly you have spotted. This may be an additional bug.
In Word 2010 both title style paragraphs appear in the navigation pane.
Second, it would be preferable to use the Heading 1 style. The title style was designed to be used for a single title for the document. If the Heading 1 style is used, this anomaly in the Document Map does not appear.
Why use Microsoft Word’s built-in heading styles? by Shauna Kelly