Model baseline comparison shouldn't allow duplicates

The model config parameter comparison_blob_storage_paths takes a list of tuples, (name, container). The name is purely descriptive , and has no further meaning, but it is important that they are distinct otherwise multiple datasets from different containers may be merged in an unpredictable way. Essentially, it is better if this is a dict[name, container]. Add this validation to the validate function.

AB#4149