VARYDEDUP default has been changed from NO to YES because VARYDEDUP=YES is one of best practices on 12.5.
When you run MIM 12.5 with SETOPT VARYDEDUP=NO, you may receive a following message by MIM_GTAF_VARY_DEDUP health check.
MIMH2006E The mimproc address space is running with SETOPTION
VARYDEDUP=OFF. Excessive duplicate vary commands can lead to tape
device allocation delays and processing overhead.