contentpropset should have the explicit bootstrap option to indicate that it is multiple or single valued

<sv:property sv:name="hippo:multiple" sv:type="Boolean">
   <sv:value>true</sv:value>
</sv:property>

    public PostStartupTask execute() throws RepositoryException {
        if (propexists) {   
            // do stuff but keep the original property (single or multiple)
        } else {
            // find out what to write: single or multiple values
        }

    public PostStartupTask execute() throws RepositoryException {
        if (explicit-bootstrap-multi-or-single) {
           // write property according explicit hippo:multiple true or false indication
        } else  if (propexists) {   
            // do stuff but keep the original property (single or multiple)
        } else {
            // find out what to write: single or multiple values
        }