some dummy content ${config1.first.placeholder}. other context on same line ${config1.second.placeholder}
some dummy content ${config1.third.placeholder}. other context on same line ${config1.duplicate.placeholder}
some dummy content ${config1.duplicate.placeholder}. other context on same line ${config1.last.placeholder}

no need to count! There are in total 5 unique placeholders:
config1.first.placeholder
config1.second.placeholder
config1.third.placeholder
config1.duplicate.placeholder
config1.last.placeholder