JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
    <textarea id="fld" class="field"></textarea>
    <button id="btn" class="button" onclick="cleanUp();">Remove</button>
</div>
<!-- end .wrapper -->

CSS

/*-----------------------------------*\
	RESET
\*-----------------------------------*/

* {

    vertical-align: baseline;

    -webkit-box-sizing: border-box;

    -moz-box-sizing: border-box;

    box-sizing: border-box;

    outline: none;

    border: none;

    margin: 0;

    padding: 0;

}

img {

    max-width: 100%;

    height: auto;

}

a {

    text-decoration: none;

}

/*-----------------------------------*\
	STYLES
\*-----------------------------------*/

body {

    background-color: #fff;

    font: 100%/1.8'Helvetica Neue', serif;

    -webkit-font-smoothing: antialiased;

    text-rendering: optimizeLegibility;

}

.wrapper {

    max-width: 640px;

    width: 640px;

    margin: 50px auto;

    position: relative;

}

.field {

    width: 640px;

    height: 345px;

    resize: none;

    background-color: #eaeaea;

    border-radius: 3px;

    padding: 10px;

    font-size: 12px;

    font-family:'Monaco';

    color: #444;

}

.button {

    background-color: #333;

    border-radius: 3px;

    padding: 13px 20px;

    cursor: pointer;

    color: #fff;

    text-transform: uppercase;

    margin: 15px 0 0 0;

}

JavaScript

function cleanUp() {
    var text = document.getElementById("fld"),
        textVal = text.value,
        array;
    
    textVal = textVal.replace(/\r/g, " ");
    array = textVal.split(/\n/g);

    text.value = removeDuplicatesInPlace(array).join("\n");
}

var removeDuplicatesInPlace = function (arr) {
    var i, j, cur, found;
    for (i = arr.length - 1; i >= 0; i--) {
        cur = arr[i];
        found = false;
        for (j = i - 1; !found && j >= 0; j--) {
            if (cur === arr[j]) {
                if (i !== j) {
                    arr.splice(i, 1);
                }
                found = true;
            }
        }
    }
    return arr;
};