What is the effective difference between these two key elements
orderings?

ALTER TABLE T ADD CONSTRAINT U UNIQUE (PK, HIGH_MAX_DUP)

ALTER TABLE T ADD CONSTRAINT U UNIQUE (HIGH_MAX_DUP, PK)

The first version is how I normally order key elements, from most unique
to least. Since a primary key index exists, the second would allow
HIGH_MAX_DUP to be used in queries. But would ordering the key elements
like this have any impact due to the high number of duplicates (it's
almost as bad as an even split between Boolean values).