If you're using NGK's for example
NGK BR 8ES are resistor plugs - no cap needed.
NGK BP 8ES are longer electrodes for inverted positions to prevent fouling.
NGK B 8ES - standard plug no resistor
If you have resistor plugs you shouldn't have a cap - double resistance/weaker spark.
Could just be a mix up last time the plugs were changed (part numbers similar) or there could be a genuine reason for this set up. I'd suggest getting in touch with whoever serviced it last and find out.
Fargoo.