When you merge 2 duplicate individual records (using "Find and Merge Duplicate Contact Records under the Contacts menu) together that both contain "Codes Redeemed", that CiviDiscount data is completely lost/deleted and NOT merged into the final record.
Here is the merge screen, notice there is nothing about merging the CiviDiscount data:
![cividiscount-mergescreen](https://cloud.githubusercontent.com/assets/16328640/11908709/6884f494-a593-11e5-9fa0-a5ac74ba6cfa.gif)
Here is what MySQL says BEFORE the merge:
mysql> select id,item_id,contact_id,contribution_id from cividiscount_track where contact_id IN (66855,2492);
+------+---------+------------+-----------------+
| id | item_id | contact_id | contribution_id |
+------+---------+------------+-----------------+
| 157 | 28 | 2492 | 16023 |
| 757 | 73 | 2492 | 17309 |
| 1501 | 73 | 66855 | 18229 |
+------+---------+------------+-----------------+
3 rows in set (0.00 sec)
Here is what MySQL says AFTER the merge:
mysql> select id,item_id,contact_id,contribution_id from cividiscount_track where contact_id IN (66855,2492);
+-----+---------+------------+-----------------+
| id | item_id | contact_id | contribution_id |
+-----+---------+------------+-----------------+
| 157 | 28 | 2492 | 16023 |
| 757 | 73 | 2492 | 17309 |
+-----+---------+------------+-----------------+
2 rows in set (0.00 sec)
If you check the Discount Code usage in the CiviCRM Discount Codes Page, it also has no record of the usage, example: Before the merge it would say 2/10 codes used, but AFTER the merge is reports 1/10 codes used.
CiviCRM 4.4.20
CiviDiscount 2.6
Drupal 6.37