Defining a "one-to-one-to-one" relationship on Firebird 2.5 EF Core

Question

I'm working with EF Core and a third-party Firebird 2.5 database and, for some reason, they decided to, rather than do a simple one-to-one relationship, create a single table with two columns that do that relationship itself, i.e.

STOCK
========
ID_STOCK(int)
more columns (and their datatypes)

STOCK_IDENTIFIER
========
ID_STOCK (int)
ID_IDENTIFIER (int)

STOCK_PRODUCT
========
ID_IDENTIFIER (int)
more columns (and their datatypes)

So, every STOCK has one STOCK_IDENTIFIER, which, in turn, has one STOCK_PRODUCT. Usually, when I'm creating my own DB in MySQL, I just set foreign keys with data annotations (I'm not fluent in Fluent API, pun intended) and let the migration do its job. However, in this case I can't change the DB's schema (on the account of it being third-party), so I need to use the existing structure.

Right now I have the following:

    public class STOCK
    {
        [Key]
        public int ID_STOCK { get; set; }
        [MaxLength(50)]
        public string DESCRIPTION { get; set; }
        [Column(TypeName = "decimal(18, 4)")]
        public decimal SELL_PRICE { get; set; }
        public STOCK_IDENTIFIER STOCK_IDENTIFIER{ get; set; }
    }

    public class STOCK_IDENTIFIER
    {
        [ForeignKey("ID_STOCK")]
        public STOCK ID_STOCK { get; set; }
        public STOCK_PRODUCT ID_PRODUCT { get; set; }
    }

    public class STOCK_PRODUCT
    {
        [ForeignKey("ID_PRODUCT")]
        public STOCK_IDENTIFIER ID_IDENTIFIER{ get; set; }
        [MaxLength(18)]
        public string GTIN{ get; set; }
        [MaxLength(18)]
        public string SKU{ get; set; }
        [Column(TypeName = "decimal(18, 4)")]
        public decimal INSTOCK_AMNT { get; set; }
    }

I read at The property X is of type Y which is not supported by current database provider that Fluent API could fix that, however, that article works flawlessly for one-to-one. As soon as I try to implement Fluent on a cascading relationship like this one, I get

            modelBuilder.Entity<STOCK_IDENTIFIER>()
                .HasOne<STOCK_PRODUCT>(p => p.ID_IDENTIFIER)
                .WithOne();

            modelBuilder.Entity<STOCK>()
                .HasOne<STOCK_IDENTIFIER>(p => p.IDENTIFICADOR)
                .WithOne();

The property or navigation 'ID_IDENTIFIER' cannot be added to the entity 
type 'STOCK_PRODUCT' because a property or navigation with the 
same name already exists on entity type 'STOCK_PRODUCT'.

Any hints on what I've been doing wrong?