If I get you correctly, you expect Oracle to ignore old duplicate values and allow new values only when they satisfy the constraint. The error is returned because when you add a UNIQUE
constraint, Oracle creates unique index on the column to check the values, but your table already have duplicate values, so it fails. I would create the non-unique index first, then add the constraint so that it uses your existing non-unique index instead of automatically creating the unique index which would fail:
create index arc_ix on arc (name);
alter table arc add constraint arc_uq unique (name) enable novalidate;