cities.js

const cities = ['Tokyo', 'London', 'Rome', 'Donlon', 'Kyoto', 'Paris'].map(city => city.toLowerCase());

const matches = cities.reduce((acc, curr) => {
	const double = curr + curr;
	const match = [];
	cities.forEach(city => {
		if(double.includes(city)) {
			match.push(city);
		}
	});
	acc.push(JSON.stringify(match.sort()));
	return acc;
}, [])

const matchesSet = new Set(matches);

const result = Array.from(matchesSet).map(pair => JSON.parse(pair));

console.log(result);