If Not lsAuthors.Contains(newAuthor) Then
    lsAuthors.Add(newAuthor)
End If

或者使用LINQs Enumerable.Any：

Dim authors = From author In lsAuthors Where author = newAuthor
If Not authors.Any() Then
    lsAuthors.Add(newAuthor)
End If

您还可以使用高效的HashSet(Of String)而不是不允许重复的列表,如果字符串已经在集合中,则在HashSet.Add中返回False.

Dim isNew As Boolean = lsAuthors.Add(newAuthor)  ' presuming lsAuthors is a HashSet(Of String)