Error:
RMAN-05158: WARNING: auxiliary (datafile) file name E:\ORADATA\ORCL\SYSAUX01.DBF conflicts with a file used by the target database

Cause or warning:
RMAN is attempting to use the specified file name as a restore destination in the auxiliary database, but this name is already in use by the target database.

Solution:

If you do not use the NOFILENAMECHECK command in RMAN for a restore with the duplicate command then this warning occurred. Example of using NOFILENAMECHECK while restoring or creating the Dataguard environment

C:\>rman auxiliary sys@prim_dr

spool log to F:\RMANSTANDBY\mylog.log; 

run {
allocate auxiliary channel t1 type disk;
allocate auxiliary channel t2 type disk;
allocate auxiliary channel t3 type disk;
allocate auxiliary channel t4 type disk;
allocate auxiliary channel t5 type disk;
allocate auxiliary channel t6 type disk;
duplicate target database for standby backup location 'F:\RMANSTANDBY\' nofilenamecheck;
release channel t1;
release channel t2;
release channel t3;
release channel t4;
release channel t5;
release channel t6;
}

In Oracle 19c, this warning occurred with NOFILENAMECHECK” is used. So, You can safely ignore it while executing the duplicate commands in RMAN.
So run the duplicate with extra care to protect your target database.