Dapper multimapping and left join

Given this test:

    public class Author
    {
        public int AuthorId { get; set; }
        public List<Book> Books { get; set; } = new List<Book>();
    }
    public class Book
    {
        public int BookId { get; set; }            
    }
    [Fact]
    public async Task DapperCollapseLeftJoin()
    {
        var sql = @"      select 1 as AuthorId, 1 as BookId
                    union select 1 as AuthorId, 2 as BookId
                    union select 2 as AuthorId, 3 as BookId
                    union select 2 as AuthorId, 4 as BookId";
        var authorsWithBooks = 
            (await AC.OpenConnection.QueryAsync<Author, Book, Author>(sql,
                (author, book) => { author.Books.Add(book); return author; }, 
                splitOn: "AuthorId, BookId")).ToList();

        // fails because we're getting 4 author+book rows
        Assert.Equal(2, authorsWithBooks.Count);
    }

The documentation for Dapper says multimapping maps rows to multiple objects and doesn't mention anything about actually collapsing objects (like would happen for left/inner joins where leftmost column values repeat).

Is there a Dapper-native way to make this happen?