Accept identical repeated named type definitions when reading (like Java Avro 1.12) #708

Problem

Some Avro writers repeat the full definition of a named type instead of referring to it by name after the first use. For example, PyIceberg and iceberg-rust 0.21 do this for two Iceberg partition fields that are both decimal(10,2):

{"type": "record", "name": "r", "fields": [
  {"name": "a", "type": {"type": "fixed", "name": "decimal_10_2", "size": 5, "logicalType": "decimal", "precision": 10, "scale": 2}},
  {"name": "b", "type": {"type": "fixed", "name": "decimal_10_2", "size": 5, "logicalType": "decimal", "precision": 10, "scale": 2}}
]}

The spec doesn't allow this, but these files already exist and won't go away until someone rewrites them. In 0.22:

let schema = Schema::parse_str(REPEATED)?;  // Ok
ResolvedSchema::try_from(&schema)?;         // Err: Two named schema defined for same fullname: decimal_10_2.

Reader::new on a file with this schema in its header fails with the same error, unless a reader schema is passed.

What Java does

Java Avro 1.12 accepts a repeat if it's identical to the first definition, and only rejects one that differs (ParseContext.put):

if (alreadyKnownSchema != null) {
  if (!schema.equals(alreadyKnownSchema)) throw new SchemaParseException("Can't redefine: " + fullName);
}

Proposal

During name resolution, treat a repeated definition that's equal to the first one as a reference, and keep the error when it differs. That's lenient on read, like Java, and a repeat that differs is still caught as genuinely ambiguous.

Why

iceberg-rust upgraded to 0.22 in apache/iceberg-rust#3354. To keep reading existing manifests, we had to add a workaround that rewrites the schema JSON in the file header (repeats become references) and reads the file again (named_types.rs). With this change, plus #575 for the other custom code, we could move all of it into avro-rs. More background in apache/iceberg-rust#3466.