coco format inconsistencies

[Eldies]

Coco does not support several annotations with the same id: only one of the annotations is accessible with pycocotools:
https://github.com/cocodataset/cocoapi/blob/master/PythonAPI/pycocotools/coco.py#L98

Datumaro can import several annotations with the same id (which is inconsistent with pycocotools behaviour).

Annotations with the same id are imported as belonging to the same group, and on export they are merged into one annotation which covers all the source annotations, i.e. importing and then exporting changes annotations, which is unexpected behaviour

datum convert -if "coco" -f "coco" merges annotations with the same id.

Here is a dataset, which can be used to illustrate the problem
Archive.zip