A weird workaround I created when I was 14 years old, kept here only for sentimental value

GenericTable.cs

using System.Collections.Generic;

public class GenericTable<T, U> {
	private List<GenericTableEntry> Table = new List<GenericTableEntry>();
	
	public void Add(T Key, U Value) {
		Table.Add(new GenericTableEntry(Key, Value));
	}
	
	public GenericTableEntry[] GetAll() {
		return Table.ToArray();
	}
	
	public U[] GetValues(T Key) {
		List<U> Entries = new List<U>();
		foreach(GenericTableEntry Entry in Table) if(Entry.GetKey().Equals(Key)) Entries.Add(Entry.GetValue());
		return Entries.ToArray();
	}
	
	public T[] GetKeys(U Value) {
		List<T> Entries = new List<T>();
		foreach(GenericTableEntry Entry in Table) if(Entry.GetValue().Equals(Value)) Entries.Add(Entry.GetKey());
		return Entries.ToArray();
	}
	
	public void Add(GenericTableEntry[] Entries) {
		Table.AddRange(Entries);
	}
	
	public class GenericTableEntry {
		private T Key;
		private U Value;
		
		public GenericTableEntry(T Key, U Value) {
			this.Key = Key;
			this.Value = Value;
		}
		
		public T GetKey() {
			return Key;
		}
		
		public U GetValue() {
			return Value;
		}
	}
}

you lied, it's not a hashtable, just a table. needs more trees.

I tried to duplicate C#'s "Hashtable.cs" that allows duplicate keys (also note the change of script name from "DuplicateHashtable" to "GenericTable")