Should I cap and seal an old and unused boiler flue totally airtight?

I have an old ceramic boiler flue in my chimney that has been taken out of service. All boiler and water heater venting is now going through the second flue next to it. I have researched at length and found a lot of conflicting opinions as to whether the unused flue should be sealed airtight top and bottom. My chimney guy says it should be, to prevent condensation from the collision of warm and cold air inside it. Others say that there should always be a small opening at the top to allow any condensation to escape. Has anyone faced this issue and resolved it successfully? Any chimney experts that have an opinion? Any and all opinions would be appreciated, thanks.