Even then the boiler pressure relief is normally in the primary side of the heating circuit - very much not potable water from the mains supply.
There *is* a requirement for a double check valve on the filling loop however since that is a place where the mains is in close proximity to contaminated water in the CH system.
The trap in the boiler also functions like a siphonic flush mechanism - discharging condensate in a batch rather than as a constant dribble. That makes freezing less likely in the first place.
Subsequent traps don't matter so long as the pipe connection to them is not airtight (since then the overflow of water from a larger downstream trap could suck water from the condensate trap). That would potentially allow a path for the escape of combustion by products into the condensate pipework.
The instructions for my Viessmann said the pipe between the two check valves should be removable - which generally means a flexible. And should be removed when not in use. Wonder how many do?
Depends on how "visible" it is I expect. Some boilers have built in filling loops that can't be removed anyway.
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