Produce an error in case of duplicate parameter names

Currently, such cases do not produce any error at runtime:

```xml
<ParameterBlock Name="block">
  <IntegerParameter Name="duplicate" Signed="false" Size="8"/>
  <IntegerParameter Name="duplicate" Signed="false" Size="8"/>
</ParameterBlock>
```

even though it is forbidden by the XML schema (and correctly identified as an error by `xmllint`).