how could duplicate pkey exist in psql?

[Yan Chunlu <springrider@xxxxxxxxx>]

recently I have found several tables has exactly the same pkey,  here is the definition:

"diggcontent_data_account_pkey" PRIMARY KEY, btree (thing_id, key)

the data is like this:

   159292 | funnypics_link_point       | 41                                                                                                                     | num

   159292 | funnypics_link_point       | 40                                                                                                                     | num

I could not even update this record.

really confused about how could this happen... thanks!