JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="field">
<div id='message'></div>

JavaScript

let field = document.querySelector('#field');
let message = document.querySelector('#message');
var cities = [];

field.addEventListener('blur', function () {

    if (doChecks(field.value)) {
        cities.push(field.value);
        console.log(cities);
    } else {
        console.log('Checks failed');
    }
});

function doChecks(val) {
    if (cities.includes(val)) {
        return false;
    }

    if (cities.length > 0) {
        const last = cities[cities.length - 1];

        if (last[last.length - 1] !== val[0]) {
            return false;
        }
    }

    return true;
}