Difference between unique and non unique index

For a unique index, there is one rowid per data value.
For a nonunique index, the rowid is included in the key in sorted order, so nonunique indexes are sorted by the index key and rowid. Key values containing all nulls are not indexed, except for cluster indexes. Two rows can both contain all nulls and not violate a unique index.

Key compression cannot be used in a unique index with a single-attribute key.