fix overwriting same file multiple times when list of list passed in

src/stpipe/step.py

@@ -993,16 +993,18 @@ def save_model(
  # list of datamodels, e.g. ModelContainer
  output_paths = []
  for i, m in enumerate(model):
- idx = None if len(model) == 1 else i
- output_paths.append(
- self.save_model(
- m,
- idx=idx,
- suffix=suffix,
- force=force,
- **components,
+ # ignore list of lists. individual steps should handle this
+ if not isinstance(m, Sequence):
+ idx = None if len(model) == 1 else i
+ output_paths.append(
+ self.save_model(
+ m,
+ idx=idx,
+ suffix=suffix,
+ force=force,
+ **components,
+ )
  )
- )
  return output_paths
  else:
  # JWST SourceModelContainer takes this path