Vaccines , help please

My 18th months had his vaccines three days ago. The problems is while his doctor giving him the vaccines on his leg, the needle was broken and the liquid came out. He did not get the whole dose that he is supposed to get. Also, the doctor said she couldn't determine which vaccine it was, because the dose and color was the same with the another needle and he should be fine because he should have got them or will get them again by 24 months. My question is if he got the same vaccines before will that be enough? Why children need to take the same vaccines agin ? Also , if just small amount entered his body will it be ok ? Last , if the dose he lost was the same dose he will take at 24 month and he never take it before, will it be fine to skip it at 18 months. I am freaking out