VSAM prevents you from cataloging two objects with the same name in the same catalog. VSAM also prevents you from altering the name of an object so that its new name duplicates the name of another object in the same catalog. VSAM does not prevent duplication of names from one catalog to another however. If you have multiple catalogs, you should ensure that a data set name in one catalog is not duplicated in another catalog. The multilevel alias facility assigns a data set name to a unique catalog. However, if the number of alias levels searched is changed, it is possible that duplicate data set names could occur. If the number of alias search levels is changed, the former name cannot be located with the multilevel alias facility unless the levels are changed back. See z/OS DFSMS Managing Catalogs.

z/OS DFSMS Access Method Services Commands describes the order in which one of the catalogs available to the system is selected to contain the to-be-defined catalog entry. When you define an object, you should ensure that the catalog the system selects is the catalog you want the object entered.

Data set name duplication is not prevented when a user catalog is imported into a system. No check is made to determine if the imported catalog contains an entry name that already exists in another catalog in the system.