Covering Index on pm_block_user

While load testing a client site, I noted that pm_block_user currently has no indexes defined. There are only two columns in this table and they are almost always used together in the module, i.e. SELECT * FROM pm_block_user where author = blank and recipient IN (blank, nobody);

I think this module would benefit quite a bit from a covering index. For example, here is an explain with a query from the module and the table with a covering index as the PK:

mysql> explain SELECT COUNT(recipient) FROM pm_block_user WHERE author = 11 AND recipient IN (12);
+----+-------------+---------------+-------+---------------+---------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+-------+---------------+---------+---------+-------------+------+-------------+
| 1 | SIMPLE | pm_block_user | const | PRIMARY | PRIMARY | 8 | const,const | 1 | Using index |
+----+-------------+---------------+-------+---------------+---------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

However, I'm not sure if this index can be the PK. Judging by pm_index, there maybe a chance for duplicate records in this table as well. Is this true? I have two patches in the wings, one a regular index and one a PK.