When you store the data for a hash table item, if the item already exists, then the data for that item is overwritten by the newest entry.
Eg: /hadd Table Name Description stores the description for the specified name in the Table
/hadd Table Name Description2 will overwrite the first Description with the second description
/hadd Table Name Description3 will overwrite the 2nd Description with the 3rd Description.

If any of the descriptions match the previous description, then you won't notice a change, although the information will have changed.

Note that Table and Name (in my above examples) are case-insensitive