Isograms programming exercise

I am sergeant Sql Pavlov, and this are your orders:

Implement a function that determines if a given string have duplicate characters, or all the characters are unique.

Example:
input: "aabc" => it have two 'a' so its not unique.
input: "abcd" => there is no repeating characters.

As always there is many way to solve this coding problem, and the solution I will provide, will not be the fastest, or even will follow the best practices, because my goal is to choose a solution that will help beginners to develop better understanding of the programming languages. I use C# because is a C based language, and also is very modern widely used and somewhat easy to start with.

Scroll down for the solution. Good luck and have fun 🙂

An Isogram is a word with no repeating letters

Which make it perfect for naming our function

    public static bool IsIsogram(string str)
    {
    //We are going to use ToLower() on "str" 
    //in order to make all character lowercase
        str = str.ToLower();

    //then we shall go through each character
    //and check its occurrences count
        foreach (var item in str)
        {
     //we will use "k" as a counter
            int k = 0;
     //now we will go through each character
     //of the string "str" 
            for (int i = 0; i < str.Length; i++)
            {
    //and check if the characters occurs more the once
                if (item == str[i])
                {
                    k++;
     //and if it does, will know that it is not
     //an Isogram
                    if (k==2) return false;
                }
            }
        }
    //but if it occurs only 1 time it is an Isogram
        return true;
    }