VERIFIED SOLUTION i

MailStream Plus what happens if parameters are duplicated (repeated) in a job

Product affected: MailStream Plus™, all versions, all platforms
Some parameters should never be repeated in a job, for example ML TYP or ALG PP.  If they are, then the job will abend, and there will be an error message in the rpt output file (DD MSRPRPT)-
1001: Parameter Repeated
followed by the name of the parameter

Some parameters can be repeated such as ENT PT, because for example the job may have multiple entry points.  BUT, each ENT PT must have a unique entry point code in position 12-20.  If two or more have the same code, then the job will abend, and the rpt output file (DD MSRPRPT) will have-
1006: Invalid Data ENT PT code repeated

Some parameters, like BG MAX, can be used numerous times for different settings of container types and subclasses, etc.  For that reason, MailStream Plus™ (MSP) cannot  check all of the possibilities.  And it will not abend if there are duplicates, it will instead use the last one encountered.  
For example, if instead of using the USPS's default value, a job has these two parameters overwriting how many inches of mail a T tray can hold-
BG MAX T                               19.00
BG MAX T                               18.00

Then MSP will use the last one, and T-size trays will be considered full with 18 inches of mail. 
UPDATED:  May 14, 2019