handling duplicate labels when converting from EDF to Onda

[kleinschmidt]

Some EDF files we've encountered in the wild have duplicate labels/transducers. Currently, OndaEDF refuses to even plan such a file because duplicate channels are not allowed in Onda. I think we should probably relax that to a warning that becomes an error once you try to execute the plan, since the easiest way to fix this is by modifying the plan itself once it's generated; preprocessing the header requires changing the label which results in an error when executing since the label does not match the actual header.

We could also try to be clever and deduplicate things but that risks violating the assumption that only channel names explicitly listed in the label matchers will be included in the output.