RepeatDetector.Scan detects repetitions when there are none.

Those REPL tests may not be doing what you think.

Take a look at these 2 lines of your method:

            foreach(int i in sequence){
                if (i.Equals(sequence[i-1]))

The foreach will assign the value of an array element to your variable i. But on the next line, you treat i as both a value and an index. This may have appeared to work if the array contents were sequential integers starting with 1. But what if the array contained something like this: { 21, 99, 3003, 22788 }? (Try it in the REPL and see!)

But the idea of using an index is good, and while you could maintain an index in a foreach by using a separate variable, a simple for loop is more suited to indexing. Plus, you can start your index with 1 instead of 0 to insure the first comparison will be in bounds.

So if you wrote your loop like this:

    for (var i = 1; i < sequence.Length; i++)

That would produce your indexes. Then you just have to be sure you're checking values against values on the next line. So that might look like one of these:

        if (sequence[i].Equals(sequence[i-1])   // effective, but perhaps a bit esoteric
        if (sequence[i] == sequence[i-1])       // simpler and more direct

Code below this line is not "best practice":

Now, just for fun (and being deliberately esoteric), you could also complete this challenge using a while loop:

        public override bool Scan(int[] s)
        {
            int i = s.Length - 1;
            while (i > 0 && s[i] != s[--i]);
            return i > 0;
        }