The row that is being inserted (or being updated to have a new primary key) contains a duplicate value of some row that already exists, in a column or columns that are constrained to have unique values. Another cause of this error might be a locking conflict if the table lock mode is page. The new or updated row is not inserted.
Roll back the current transaction and execute it again without any duplicate rows or with the locking conflict resolved.
If you are using repeatable read isolation, then the error could be due to a unique constraint being violated. Refer to error -268.