Election Obligations & Standards Database
"In order to safeguard the ballot, each ballot should bear an official stamp specific to the polling station and/or the signature of an authorized person/s in the polling station."
Norwegian Helsinki Committee: Manual on Human Rights Monitoring: An Introduction for Human Rights Field Officers
Manual on Human Rights Monitoring: An Introduction for Human Rights Field Officers
Provided they allowed for consistency with other international obligations, safeguards may be used to diminish the potential for duplicate voting.