Duplicate rows in {location_instance} table

On my site there is are duplicate entries in table [location_instance]. How did that can happen?

To find the duplicate entries, execute:

SELECT nid, vid, uid, genid, lid FROM `location_instance` GROUP BY nid, vid, uid, genid, lid HAVING COUNT(*) > 1;
+------+------+-----+-------+------+
| nid | vid | uid | genid | lid |
+------+------+-----+-------+------+
| 4684 | 4684 | 0 | | 1792 |
| 4685 | 4685 | 0 | | 1794 |
+------+------+-----+-------+------+
2 rows in set (0.09 sec)

To see the duplicates:

mysql> SELECT * FROM `location_instance` WHERE nid in (4684, 4685);
+------+------+-----+-------+------+
| nid | vid | uid | genid | lid |
+------+------+-----+-------+------+
| 4684 | 4684 | 0 | | 1792 |
| 4684 | 4684 | 0 | | 1792 |
| 4685 | 4685 | 0 | | 1794 |
| 4685 | 4685 | 0 | | 1794 |
+------+------+-----+-------+------+
4 rows in set (0.01 sec)

As i see location_instance table has no primary key - so i think that key must be added.

And also in location_save_locations() function must have check if the same record already exists before inserting new one.