Since 4.0, the service parameters can be multiple.
The serviceParameter method of AmetysXSLTHelper should be enable to get all values, such as :

<paramName name="paramName" type="string">
      <value>value1</value>
      <value>value1</value>
      <value>value1</value>
</paramName>

For now, #serviceParameter("paramName") returns value1