Since this is actually a recent Dutch case, the primary answer will need to take into account Dutch laws.

R is guilty of the double murder. There's indeed no question about this. Even if there are other people who bear some guilt, this does not remove the guilt from R.

There is no doubt that there is a causal relationship between the call by P and the shooting. It does not matter who the victims are; the only question is whether the murder was a foreseeable consequence of the actions of P. 

It's pretty well established in Dutch law that you can't just go out in a park and shoot someone. Shootings are also pretty rare, as gun onwership is very uncommon in the Netherland. We know from the question here and the actual case that P does not know R and was therefore not aware of any mental condition of R, or anything otherwise unusual that would warrant special consideration. Therefore, P had no reason to assume that R would shoot someone, let alone two random persons. 

Since P has no reason whatsoever to expect that his actions would lead to two deaths, there is no intent to kill (not even manslaughter ("doodslag")), nor did he knowingly accept the risk ("voorwaardelijke opzet"). Even the weakest charge we can consider here ("dood door schuld"), i.e. negligent homicide requires an explicit finding of guilt (_culpa_) which is absent here.