Same ninja buff don't stack, but similiar buffs from other ninjas stack.
For example both Mifune and Iruka grant to a Midnight Blade or to an Azure Fang a +30% nin/atk. If you use both you get +60%, but two mifunes ran by you and by another player, for example in gnw, don't stack.
About pains, yes you see the buff appearing two times, but the 2nd overlays the 1st, exactly like happens with two mifunes or with two hashiarmas.