Records with same user key stored in Siebel DB

Hello All,

Could you let me know how two records with same user key can get stored in the Siebel Database.

Scenario:Table:S_ADDR_PER

BC:Personal Address

The user key configuration is based on the Address name which is a concatenation of the address type,Contact row_id,Address start date,Address end date

S_ADDR_PER already held an address record with user key as

Record 1: Residential 1-ZZZXXX 03/05/1990 00:00:00 03/05/1991 00:00:00

Record 2: Initally present in DB with user key 'Residential 1-ZZZXXX 03/05/1990 00:00:00'

User updated record 2 with an end date and the BC user property 'On Field Update' set the user key of record 2 to ' Residential 1-ZZZXXX 03/05/1990 00:00:00 03/05/1991 00:00:00'( same as record 1 user key)