bug: schema does not reject multiple field ids

[twuebi]

Bug

We allow this:

sc := iceberg.NewSchema(0,
	        ...
		iceberg.NestedField{
			Type: &iceberg.StructType{
				FieldList: []iceberg.NestedField{
					{
						Type: typ,
						ID:   6,
						Name: "inner_op",
					},
					{
						Type:     typ,
						ID:       6,
						Name:     "inner_req",
						Required: true,
					},
					...
				},
			},
			ID:       5,
			Name:     "struct",
			Required: true,
		},
                ...
	)

while java rejects this:

new Schema(
        Types.NestedField.required(1, "id", Types.LongType.get()),
        Types.NestedField.optional(2, "top", type),
        Types.NestedField.optional(3, "arr", Types.ListType.ofRequired(4, type)),
        Types.NestedField.required(
            5,
            "struct",
            Types.StructType.of(
                Types.NestedField.optional(6, "inner_op", type),
                Types.NestedField.required(6, "inner_req", type),
                Types.NestedField.optional(
                    8,
                    "struct_arr",
                    Types.StructType.of(Types.NestedField.optional(9, "deep", type))))));

  java.lang.IllegalArgumentException: Multiple entries with same key: 6=struct.inner_req and 6=struct.inner_op
...
	at org.apache.iceberg.types.IndexByName.byId(IndexByName.java:81)
	at org.apache.iceberg.types.TypeUtil.indexNameById(TypeUtil.java:174)
	at org.apache.iceberg.Schema.lazyIdToName(Schema.java:225)
	at org.apache.iceberg.Schema.<init>(Schema.java:154)

We could probably verify this in the same place as java, in schema.init()