In this example, we create an HTML form that includes a textarea for users to input their text, a button to trigger the checking function, and a div element to display the results. We then define a JavaScript function called checkDuplicates() that retrieves the input text, splits it into individual sentences based on the period character, and checks for duplicates by comparing each sentence with all subsequent sentences in the array.

Duplicate Sentence Checker

Enter some text below and click the button to check for duplicate sentences:

If any duplicates are found, they are added to a separate array, and the function outputs a message to the user indicating which sentences are duplicated. If no duplicates are found, the function simply outputs a message indicating as much.

Note that this is just one possible implementation of a Duplicate Sentence Checker, and there are many ways to modify or enhance this code to suit your specific needs.