MySQL - Come unire due tabelle senza duplicati?

Prova questo:

SELECT hotels.hotelID, 
hotels.hotelName,
GROUP_CONCAT(operators.opName SEPARATOR ', ') AS opList
FROM hotels
INNER JOIN operators 
ON operators.opHotelID = hotels.hotelID
GROUP BY(hotels.hotelID)

Se vuoi avere il numero di operatori, devi utilizzare COUNT sull'ID operatori in questo modo:

SELECT hotels.hotelID, 
hotels.hotelName,
GROUP_CONCAT(operators.opName SEPARATOR ', ') AS opList,
COUNT(operators.opID) AS nbOperatos
FROM hotels
LEFT JOIN operators 
ON operators.opHotelID = hotels.hotelID
GROUP BY(hotels.hotelID)