Correct representation of Respository Pattern

Normally, I have implemented Repository Pattern using EF. Here I am using list. Can you guys identify any pitfalls here please. Anywhere it can break. Is it okay to check for null in every function itself

public class Repository<T> : IRepository<T>
{
    private List<T> storeDatabase;

    public Repository()
    {
        storeDatabase = new List<T>();
    }
    public IEnumerable<T> All()
    {
        return storeDatabase;
    }

    public void Delete(IComparable id)
    {
        storeDatabase.RemoveAll(a => a.Id.Equals(id));
    }

    public T GetById(IComparable id)
    {
        return storeDatabase.FirstOrDefault(i => i.Id.Equals(id));
    }

    public void Add(T item)
    {
        if (item==null)return;

        storeDatabase.RemoveAll(a => a.Id.Equals(item.Id));
        storeDatabase.Add(item);
    }

}