Unique Constraint now uses non unique backing indexes with new attribute UniqueWithDuplicateNulls. This index has following sharing properties
1. Can use an existing unique index.
2. Non Unique indexes (and foreign key) can use this index.
While dropping a unique index a new index for unique constraint should be created.
when a unique constraint is dropped a new index for a non unique index or foreign key should be created.
Army has found several issues in actual behavior. This problems are listed in