ArrayFound.cs

// a - element
// c - amount
// answer is - p1.a * p1.c + p2.a * p2.c
// Invariant: p2.a == p3.a
using Triple = ((int a, int c) p1, (int a, int c) p2, (int a, int c) p3);

internal static class SubArray
{
    internal static int Solve(int[] xs)
    {
        Triple start = ((0, 0), (0, 0), (0, 0));
        return xs.Scan(start, Next)
                 .Max(t => t.p1.a * t.p1.c + t.p2.a * t.p2.c);
    }

    private static Triple Next(Triple prev, int x)
    {
        (var p1, var p2, var p3) = prev;
        if (p1.a == x)
            return (p2, p1.Increment(), x.Single());

        if (p2.a == x)
            return (p1, p2.Increment(), p3.Increment());

        return (p3, x.Single(), x.Single());
    }

    private static (int a, int c) Single(this int x) => (x, 1);
    private static (int a, int c) Increment(this (int a, int c) p) => (p.a, p.c + 1);

    // Scan [1,2,3,4,5] 0 (+) => [0,1,3,6,10,15]
    internal static IEnumerable<TAccumulate> Scan<TSource, TAccumulate>(
        this IEnumerable<TSource> source,
        TAccumulate seed,
        Func<TAccumulate, TSource, TAccumulate> func)
        {
            yield return seed;
            foreach (var item in source)
            {
                seed = func(seed, item);
                yield return seed;
            }
        }
}

public class ArrayTest
{
    [Theory]
    [InlineData(0       /*empty array*/)]
    [InlineData(4,      1, 1, 1, 1)]
    [InlineData(7,      1, 2, 3, 4)]
    [InlineData(7,      1, 2, 2, 1, 1, 3, 1, 1)]
    [InlineData(8,      1, 2, 2, 1, 1, 3, 3)]
    [InlineData(9,      1, 2, 2, 1, 1, 1, 3, 6)]
    [InlineData(8,      1, 2, 2, 1, 1, 1, 3, 2)]
    public void Test(int e, params int[] a)
    {
        Assert.Equal(e, SubArray.Solve(a));
    }
}

По мотивам статьи на Habr тоже решил попробовать решить примеры.
Решение "в лоб" получается чуть эффективнее (как по скорости, так и по памяти)
https://github.com/insanity13/sandbox/blob/main/Sandbox/SubArrays/readme.md

Method	Mean	Error	StdDev	Gen0	Allocated
DimsFromDergachyVersion	27.065 ms	0.0228 ms	0.0202 ms	93.7500	839936 B
DenisNP_Version	79.534 ms	0.3393 ms	0.3174 ms	-	959584 B
MySolutionVersion	8.913 ms	0.0088 ms	0.0082 ms	-	32 B