Prebent multiple XDR mappings with the same transducer name

[wellenvogel]

Currently the api function addXdrMapping will allow you to add an xdr mapping with a transducer name that already exists (with other type/unit). This should be prevented as those mappings would end up on the same boat data value.
But it still should be possible to add a mapping with the same type/transducerName but a different unit.