This is a small query improvement to accessible_ids() that removes

the usage of DISTINCT when querying a list of accessible IDs for

Group and Repository. This means less work needed by the database,

removing the usage of a temporary table on MySQL. Any de-duplication

will now be done in Python, by converting to a set first.
It's a small performance improvement setting the stage for some larger

work on improving query performance.
Both versions of accessible() still uses DISTINCT by default, but

this can be turned off by passing distinct=False.

Unit tests passed.
Compared the EXPLAIN output between the two versions of the query

on MySQL