Remove duplicates in MySQL table - set group_id when city_id is the same

I have table units in my database. In schema I have fields id, unit_id, group_id, city_id.

For simple I have 3 units:

(1, 1, 1, 1)
(2, 1, 2, 1)
(3, 1, 3, 2)

How can I remove useless groups id, when city id is the same. I have next result:

(1, 1, 1, 1)
(2, 1, 1, 1)
(3, 1, 3, 2)

I know how do this in PHP, but I think 'maybe MySQL has inbuild functions which i don't know' ;)

Regards