If my infant son received his four month old shots twice (not my doing btw), will that harm him? nurse got confused and gave shots when not supposedto

No harm : If the child is healthy , it is probably not harmful although not needed

Unlikely to be a pbm: Other than the discomfort felt initially the extra will have no negative effects. The regular dose size is chosen as the smallest amount that gives the desired stimulation to the babies immune system.The higher dose may push response up more, but the effect would be short lived.