IEquatable

using System;
using System.Collections.Generic;

class MyBase : IEquatable<MyBase>
{
    public string X { get; set; }
    public override bool Equals(object other)
    {
        MyBase o = other as MyBase;
        return (o != null) && Equals(o);
    }
    public bool Equals(MyBase other) => this.X == other.X;

    public override int GetHashCode()
    {
        var hc = new HashCode();
        hc.Add(X);
        return hc.ToHashCode();
    }
}

// Uncomment to make the repro fail
class MyItem : MyBase // , IEquatable<MyItem>
{
    public string Y { get; set; }

    public override bool Equals(object other)
    {
        MyItem o = other as MyItem;
        return (o != null) && this.X == o.X && this.Y == o.Y;
    }

    public bool Equals(MyItem other) => base.Equals((MyBase)other);

    public override int GetHashCode()
    {
        // var hc = new HashCode();
        // hc.Add(base.GetHashCode());
        // hc.Add(Y);
        // return hc.ToHashCode();

        // Return poorly distributed hashcode to make the repro fail predictably
        return 1;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var d = new Dictionary<MyItem, int>()
        {
            { new MyItem() { X = "A", Y = "B" }, 1 },
            { new MyItem() { X = "A", Y = "C" }, 2 }
        };
    }
}