Reproducer:

 $ jfr configure --ouput a.jfc
 $ jfr configure --ouput b.jfc
 $ jfr configure --input a.jfc,b.jfc --output combined.jfc

Expected:

jfr configure: parsing error: Control with 'gc' is declared in multiple files

Was:
Control with 'gc' is declared in multiple files
at jdk.jfr/jdk.jfr.internal.jfc.model.JFCModel.<init>(JFCModel.java:69)
at jdk.jfr/jdk.jfr.internal.tool.Configure.configure(Configure.java:198)
at jdk.jfr/jdk.jfr.internal.tool.Configure.execute(Configure.java:180)
at jdk.jfr/jdk.jfr.internal.tool.Main.main(Main.java:87)
jfr configure: parsing error: Control with 'gc' is declared in multiple files