Yes there are times when a extra expansion vessel would be placed on a heating system in addition to the expansion vessel fitted in the combination boiler
The size of the expansion vessel placed on a system is calculated by the amount of water in the system,the temperature the water will be heated to and the maximum acceptable expansion in the system
All combination boilers you purchase have an expansion vessel fitted that assumes that the boiler will be the only heat source on the system, that the water in the system will be heated to its capabilities only and placed on a system that can be covered by its output,once you vary away from the boiler spec,you have to adjust likewise,even if you have a normal system with a combination boiler at its/ but within its maximum limits you may have to fit an additional expansion vessel to reduce system water expansion and add a reasonable tolerance
After saying above,if you have asked the question because you have come across an external expansion vessel next to the combination boiler,it could be that the one on the combination boiler is damaged and the external vessel has been added to replace the internal one .A quick alternative to removing boiler from wall to replace internal vessel,also normal expansion vessel usually cheaper to purchase
