You can define the primary index as unique (UPI) or nonunique (NUPI), depending on whether duplicate values are allowed in the indexed column set.
UPIs provide optimal data distribution and are typically assigned to the primary key for a table. However, the primary key is not necessarily always the best candidate for a primary index.
When a table is designed with a NUPI, consider assigning a uniqueness constraint such as PRIMARY KEY or UNIQUE to the primary or other alternate key of the table. This both enforces uniqueness and enhances retrieval for applications where the primary or other alternate key is frequently used as a selection or join criterion.