this is already raised there as an issue

.

How about instead raise its difficulty?

Typo in description : "locigal" instead of "logical".

C#

In Solution section: typo in the method comment : "// retrun" > "// return".

As of today the description says "there are no duplicates".

done, and i added an assertion message with the input as well.

C#

The method names should start with capital letter : "Test1", "Testing".

Each assert should be in separate test method.
Like so:

        [Test]
        public static void Test1()
        {
            Assert.That(DoubleLinear.DblLinear(10), Is.EqualTo(22));
        }
        
        [Test]
        public static void Test2()
        {
            Assert.That(DoubleLinear.DblLinear(20), Is.EqualTo(57));
        }
        
        [Test]
        public static void Test3()
        {
            Assert.That(DoubleLinear.DblLinear(30), Is.EqualTo(91));
        }
        
        [Test]
        public static void Test4()
        {
            Assert.That(DoubleLinear.DblLinear(50), Is.EqualTo(175));
        }

So that results look like this:

     Test1 Success
     Test2 Failed:   Expected: 57
  Expected: 57
  But was:  55

   at Twice_linear___tests.Tests.Test2() in C:\Users\...\UnitTest1.cs:line 16



     Test3 Failed:   Expected: 91
  Expected: 91
  But was:  121

   at Twice_linear___tests.Tests.Test3() in C:\Users\...\UnitTest1.cs:line 22



     Test4 Failed:   Expected: 175
  Expected: 175
  But was:  171

   at Twice_linear___tests.Tests.Test4() in C:\Users\...\UnitTest1.cs:line 28

A shorter way:

        [TestCase(10, 22)]
        [TestCase(20, 57)]
        [TestCase(30, 91)]
        [TestCase(50, 175)]
        public static void Test(int actual, int expected)
        {
            Assert.That(DoubleLinear.DblLinear(actual), Is.EqualTo(expected));
        }

Produces this result:

    Test(10,22) Success
    Test(20,57) Failed:   Expected: 57
Expected: 57
But was:  55

 at Twice_linear___tests.Tests.Test(Int32 actual, Int32 expected) in C:\Users\...\UnitTest1.cs:line 38



    Test(30,91) Failed:   Expected: 91
Expected: 91
But was:  121

 at Twice_linear___tests.Tests.Test(Int32 actual, Int32 expected) in C:\Users\...\UnitTest1.cs:line 38



    Test(50,175) Failed:   Expected: 175
Expected: 175
But was:  171

 at Twice_linear___tests.Tests.Test(Int32 actual, Int32 expected) in C:\Users\...\UnitTest1.cs:line 38

Why does the example not mention "1 2 4 6 8" as third way to decompose 11?

For C# the opening and closing braces of the class should be at the line start.

Typo in the description in "The digits are sequential, incementing†: 1234" : "incementing" instead of "incrementing".

In C# it expects an array where 3rd element is expected 1 for true, 0 for false, what is your language?

It does : "returns an array/tuple (check the function signature/sample tests for the return type in your language".

For C# method name productFib should start with capital letter - ProductFib.