call is ambiguous between the following methods System.Linq.Enumerable.ToList()

My asp.net solution has a class library that targets framework net 4.62. I'm getting an error when I call .ToList().

The call is ambiguous between the following methods or properties: System.Linq.Enumerable.ToList<TSource>(System.Collections.Generic.IEnumerable<TSource>) and System.Linq.Enumerable.ToList<TSource>(System.Collections.Generic.IEnumerable<TSource>)

Reading through similar errors on SO, I can conclude that I have duplicate code in my files. I searched through the assembly in Object Explorer and found that they are located in both System.Core[4.0.0.0] and System.Core[5.0.5.0].

Here's the class:

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;

namespace PortalRepository.Implementation.Repositories
{
    public class PortalRepository<TEntity>: IPortalRepository<TEntity> where TEntity:class 
    {
        internal readonly PORTAL_Entities _context;
        internal IDbSet<TEntity> _dbSet;

        private IDbSet<TEntity> Entities
        {
            set { }
            get { return _dbSet ?? (_dbSet = _context.Set<TEntity>()); }
        }
        public IList<TEntity> AllEntities
        {
            set { }
            get { return Entities.ToList(); //ambiguous call here}
        }
    }
// plus other implementation of Entities and AllEntities
}

I am getting warnings from Visual Studio 2017 that there is "No way to resolve conflict between 'System.Core, Version=4.0.0.0,...' and 'System.Core, Version=3.5.0.0,...' I thought that the above error would be related to this, but digging into Object explorer I couldn't find the method System.Linq.Enumerable.ToList() in the System.Core v3.5.0.0. So, I'm confused.

UPDATE

In this project the Dependencies are:

-Assemblies
    *System
    *System.Core
    *System.Data
    *System.Drawing
    *System.IO.Compression.FileSystem
    *System.Numerics
    *System.Runtime.Serialization
    *System.Xml
    *System.Xml.Linq
    *System.Xml.Serialization
-NuGet
    *EntityFramework(6.1.3)

END UPDATE

It has been a challenge to get this project up and going since I inherited an ASP.NET Core project and had to add .Net 4.62 class libraries to it. My knowledge of C# is growing, but is still in its infancy. Any help or guidance is much appreciated.