Extensions with conflicting fields do not provoke a warning or error

[kurtraschke]

If multiple extensions register the same field name, this leads to duplicate fields in the output. Extension registration should perform some validation to ensure that registered extensions' fields do not conflict in harmful ways.

When reading or writing, re-registering the same extension class should not be harmful (that is, onebusaway-csv-entities should do the right thing).

Arguably, registering a different extension class which defines a field with the same name and data type as an existing field is also not harmful (and thus should not provoke an error).

Originally posted by @sdjacobs in OneBusAway/onebusaway-gtfs-modules#115 (comment)