A string is duplicate if the same string exists on more that one location in the project. If one of those rows has a value, it is copied to the other rows. If the string has been translated in two or more different way the Select Translation dialog box is shown. It lets you select the translation that is used.
When you type a translation Sisulizer automatically translates the duplicate strings.
Duplicate string group is a collaction of duplicate strings that have the same original value and the same translations.