Not likely, 15 amp duplex receptacle are manufactured with the knowledge they may be protected by a 20 amp OCPD.
Originally Posted by wec
It is done all the time and it is safe.
Certainly if there is load for each phase the overall neutral current will be reduced but I don't think its wise to rely on this...
The neutral must be sized large enough for the maximum current that can be applied to it.
In most cases that means a one or two size reduction, in other cases it may be reduced much more if the majority of loads are line to line.
Most systems work best if they are kept simple rather than made complicated; therefore simplicity should be a key goal in design and unnecessary complexity should be avoided.