const strings = ['russia', 'usa', 'russia', 'france', 'russia', 'spain'];

let result = [];

​

for(let i = 0; i < 100; ++i) {

  const cities = strings.reduce((acc, item) => {

    return [...acc, item];

  }, []);

  [...new Set(cities)];

}

​

const strings = ['russia', 'usa', 'russia', 'france', 'russia', 'spain'];

​

for(let i = 0; i < 100; ++i) {

  const cities = strings.reduce((acc, item) => {

    if (!acc.includes(item)) {

        return [...acc, item];

    }

    return acc;

  }, []);

}

​