By default, properties are multivalued, there are no restrictions in the number of values a property can store.

INSERT DATA { <cat> a ex:Mammal .
              <dog> a ex:Mammal .

              <peter> a ex:Mammal ;
                    ex:pets <cat>, <dog> }

Wherever this is not desirable, cardinality can be limited on properties through nrl:maxCardinality.

ex:cromosomes a rdf:Property;
              rdfs:domain ex:Eukaryote;
              rdfs:range xsd:integer;
              nrl:maxCardinality 1.

This will raise an error if the SPARQL updates in the endpoint end up in the property inserted multiple times.

# This will fail
INSERT DATA { <cat> a ex:Mammal;
                    ex:cromosomes 38;
                    ex:cromosomes 42 }

# This will succeed
INSERT DATA { <donald> a ex:Mammal;
                       ex:cromosomes 47 }