Thank you Gail.

I did it like:

DELETE ACT_SECUNDARIAS FROM ACT_SECUNDARIAS AS a INNER JOIN actividades_secundarias AS b

ON a.NIF_ANTIGO = b.NIF_ANTIGO and a.COD_CAE = b.COD_CAE and a.COD_SECTOR = b.COD_SECTOR and a.DT_BEGIN = b.DT_BEGIN

WHERE b.rn >1