Questa qui:

delete t1 from nome_tabella as t1, nome_tabella as t2
where t1.attr1 = t2.attr1 and
t1.attr2 = t2.attr2 AND
t1.attr3 = t2.attr3 AND
t1.attr4 = t2.attr4 AND
t1.attr5 = t2.attr5 AND
t1.attr6 = t2.attr6
and t1.attr7 < t2.attr7