pg_statistic corruption and duplicated primary keys

Lately my database crashed and I've had some strangeness following.

I found that some tables would have two distinct rows with identical
primary key.

Now, VACUUM complains thusly

WARNING: index "pg_statistic_relid_att_index" contains 2984 row
versions, but table contains 2983 row versions

HINT: Rebuild the index with REINDEX.

After trying to reindex, I get

ERROR: could not create unique index

DETAIL: Table contains duplicated values.

I think I'm seeing the same issue - a duplicated row.

I know this should not happen - but does it in fact come to happen
sometimes?

Is there a smarter way of dealing with it than deleting the duplicate?

How can these duplicates get produced?

And, for pg_statistic in particular, is it safe to muck with it? What
should I do?

Thanks

Jaime