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GH-36014: [Go] Allow duplicate field names in structs (#36015)



### Rationale for this change

The package segfaulted before.

### What changes are included in this PR?

Allow duplicate field names (in general methods will return the first field if there are duplicates)

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes

* Closes: #36014

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>