Looking at the current function:

 def add_class_key_required_unique_constraint(self, cn, key):
        sql = '''create unique index _%s_key_retired_idx
            on _%s(__retired__, _%s)'''%(cn, cn, key)
        try:
            self.sql(sql)
        except StandardError:
            # XXX catch e.g.:
            # _sqlite.DatabaseError: index _status_key_retired_idx
already exists
            pass

it looks like it will catch and ignore that error if an exception
is raised. The checkin that added that says:

changeset:   4774:3adff0fb0207
user:        Thomas Arendsen Hein
date:        Fri Mar 22 15:53:27 2013 +0100
files:       CHANGES.txt roundup/backends/back_sqlite.py
roundup/backends/rdbms_common.py
description:
Fixed issue2550595: Allow migrating from roundup 0.x to 1.4

All changes were required to make an upgrade from 0.6 to 1.4,
The changes affecting "retired" were required for an upgrade from 0.8 to
1.4.

So I am closing this.