Relational database primary key requirements
For each relation (ie. table) the primary key value must be unique and immutable (ie. it doesn't change).

You're likely to have a relation for location, one for rooms, one for racks. The primary key values for each of them must all be unique, so two racks in different rooms would not have the same PK value. However you could have the same PK value for a rack and a location, because they are different relations. There would be no significance to the values being the same or different across relations.