Usually administering injectable drugs , especially something like morphine requires 2 nurses to cross check the order and the dose, so the second person will hopefully notice a mistake if it is made. ( And hopefully notice if the doctor has made a mistake with the prescription ) It sadly does still happen despite training and systems put in place to minimise errors and maximise chances of catching them. Most medication errors are not going to cause real harm, an extra dose of antibiotics by mistake is not likely to cause problems, but sometimes serious errors do happen and sometimes people are harmed or even die. No system is ever perfect , and all people will sometimes make mistakes, and all that can be done is to design a system that will prevent as many errors as is possible.
There are no protective systems in place for peptides, and the vials have a wide range of doses , are usually not carefully labelled and each vial has enough to make you quite ill, so it is not that surprising that accidental GLP overdoses are fairly common. There are a few posts on this forum from people who were were not newbies who have done it , but thankfully there were no major issues.