Add positive `duplicateCertsTx` tests to `Shelley` and `Conway`

[Soupstraw]

I think it would be an awesome addition if we moved this example to shelley era and added it as a positive test, where:

• In Shelley till Babbage we successfully decode such transaction, but only one certificate is preserved
• In Conway only one certificate is preserved

Originally posted by @lehins in #5370 (comment)