Including same preprocessor twice doesn't cause an error

[ledm]

I've found that including the same preprocessor twice is not causing an error in ESMValTool 2.9.0. My preprocessor was:

preprocessors:
  # --------------------------------------------------
  # Time series preprocessors
  # --------------------------------------------------
  prep_regional_ts_3d_surface: # For regional fields
    custom_order: true
    extract_region: &mpa_region {end_latitude: -50., end_longitude: -10, start_latitude: -20.,
      start_longitude: 20.}

    area_statistics:
      operator: mean

    area_statistics:
      operator: mean

    extract_levels:
       scheme: nearest_extrapolate
       levels: [0.5, ]

@valeriupredoi said that this should flag a DuplicatePreprocessurNameError, but it does not. It attempts to run the preprocessor chain and fails silently.

[ledm]

Having said that, I can envisage certain situations where you would want to run the same thing qpre processor more than once. For instance, you might want to extract a largish region, perform a regrid, then extract a smaller second region.

[bouweandela]

It attempts to run the preprocessor chain and fails silently.

Could you explain in more detail what you mean by 'fails silently'? Considering the YAML file format, it seems more likely that the second definition will overwrite the first one and then things will run as usual.