Duplicate variable names when batching inserts with only default values

[mikary]

When attempting to add multiple instances with default values to the context, a System.Data.SqlClient.SqlException is thrown on SaveChanges with the message "The variable name '@generated5' has already been declared. Variable names must be unique within a query batch or stored procedure."

Reproduction

    public class Program
    {
        public void Main(string[] args)
        {
            using (var Sql = new TestDataModel())
            {
                Sql.Database.EnsureDeleted();
                Sql.Database.EnsureCreated();

                var pictures = new List<Picture>();

                for (var index = 0; index < 15; index++)
                {
                    var picture = new Picture();
                    Sql.Pictures.Add(picture);
                    pictures.Add(picture);
                }

                for (var index = 0; index < 5; index++)
                {
                    var package = new PicturePackage
                    {
                        Pictures = new List<Picture>
                        {
                            pictures[3 * index],
                            pictures[3 * index + 1],
                            pictures[3 * index + 2]
                        }
                    };

                    Sql.PicturePackages.Add(package);
                }

                Sql.SaveChanges();
            }
        }
    }

    // Sample database with object relations.
    public class TestDataModel : DbContext
    {
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(
                new SqlConnectionStringBuilder
                {
                    DataSource = @"(localdb)\MSSQLLocalDB",
                    MultipleActiveResultSets = true,
                    InitialCatalog = "BatchingDefaults",
                    IntegratedSecurity = true,
                    ConnectTimeout = 30
                }.ConnectionString);
        }

        public virtual DbSet<Picture> Pictures { get; set; }
        public virtual DbSet<PicturePackage> PicturePackages { get; set; }
    }

    public class Picture
    {
        public int PackageId { get; set; }

        [Required]
        [ForeignKey(nameof(PackageId))]
        public PicturePackage Package { get; set; }

        [Key]
        public int Id { get; set; }

    }

    public class PicturePackage
    {
        [InverseProperty(nameof(Picture.Package))]
        public virtual ICollection<Picture> Pictures { get; set; }

        [Key]
        public int Id { get; set; }
    }

Generated SQL

SET NOCOUNT ON;
DECLARE @generated5 TABLE ([Id] int);
INSERT INTO [PicturePackage]
OUTPUT INSERTED.[Id]
INTO @generated5
DEFAULT VALUES;
SELECT [Id] FROM @generated5;
DECLARE @generated5 TABLE ([Id] int);
INSERT INTO [PicturePackage]
OUTPUT INSERTED.[Id]
INTO @generated5
DEFAULT VALUES;
SELECT [Id] FROM @generated5;
DECLARE @generated5 TABLE ([Id] int);
INSERT INTO [PicturePackage]
OUTPUT INSERTED.[Id]
INTO @generated5
DEFAULT VALUES;
SELECT [Id] FROM @generated5;
DECLARE @generated5 TABLE ([Id] int);
INSERT INTO [PicturePackage]
OUTPUT INSERTED.[Id]
INTO @generated5
DEFAULT VALUES;
SELECT [Id] FROM @generated5;
DECLARE @generated5 TABLE ([Id] int);
INSERT INTO [PicturePackage]
OUTPUT INSERTED.[Id]
INTO @generated5
DEFAULT VALUES;
SELECT [Id] FROM @generated5;

[AndriySvyryd]

Fixed in 71c1769