We might get error while creating Alternate Key as shown below.
The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name ‘dbo.sab_fash_transactionheaderBase’ and the index name ‘ndx_for_entitykey_sab_fash_headername’. The duplicate key value is (0000001414).
The reason was because there was already records created for that entity and had the field (alternate key one) having duplicate values. Deleting the duplicate record allowed us to create the alternate key on that field.
Hope it helps..