problem with replace action

i have 2 mythic mobs with the action replace but only one spawn in the world and i don't no why.
can you help me?
spawn file:
/CODE
RandomScientist:
MobType: Scienziato
Worlds: world
Change: 1
Priority: 1
Action: replace
RandomMerchant:
MobType: Merchant
Worlds: world
Change: 1
Priority: 1
Action: replace