According to
DUPLICATE SKIP TABLESPACE failes RMAN-05586 RMAN-05587 RMAN-05588 RMAN-05589 (Doc ID 1287276.1)
You MUST NOT exclude
- SYS-owned objects
- or tablespaces with rollback segments,
- nor tablespaces containing "MATERIALIZED VIEWS"!
or documentation (up to 18c) about RMAN "duplicate" command "skip tablespace" clause
https://docs.oracle.com/cd/E11882_01/backup.112/e10643/rcmsynta020.htm#r17c1-t79
https://docs.oracle.com/en/database/oracle/oracle-database/18/rcmrf/DUPLICATE.html#d43611e2528
You must not exclude SYS-owned objects or tablespaces with rollback segments, nor tablespaces containing materialized views
Lifting this restriction will significantly shorten the time in duplicating a database when you're only interested in a small set of data completely unrelated to the others that may include materialized views.
The reason for this restriction is not clear. For those interested, there's some speculation at
https://jonathanlewis.wordpress.com/2014/03/10/duplicate-database/
See the paragraph beginning with "One oddity about the whole MV thing".