Codeshare agreement

Codesharing is a type of partnership that airlines have. Airlines agree to sell each other's tickets on some routes. This means that both airlines sell more tickets.
Sometimes, airlines join groups (called alliances) which all agree to sell each other's tickets. For example, both United Airlines and Lufthansa are part of Star Alliance. This means that Lufthansa can sell tickets for United Airlines and United Airlines can sell Lufthansa's tickets.
Because airlines sell each other's tickets, some flights will have more than one flight number.