If every transaction has a "Transaction Public Key" field, why is this public key repeated on the "extra" field?

Question

I understand that the extra field in a transaction is a "freestyle" field, but after exploring several transactions on xmrchain, I noticed that the transaction public key tends to be repeated inside extra despite already being indicated in its dedicated field.

Example:

Tx hash: 397d7b4f155214420cd168810e086a6d34f92a89723b961eedb58645bf872c92

Tx prefix hash: 904b7a8b6424c708301ea903cb7a3474374f67e56f191c0a371d60a2d53a7a4c

Tx public key: 3e6442e081c5ff7e7fd9277ebd1f4f7e74bc974a53d20b36a99f5474b6424e0c

Extra: 013e6442e081c5ff7e7fd9277ebd1f4f7e74bc974a53d20b36a99f5474b6424e0c020901029c7a5118ae58f5

Any idea why does this happen?

Answer 1

If every transaction has a "Transaction Public Key" field, why is this public key repeated on the "extra" field?

The transaction public key(s) are only in the tx extra field, not repeated. Plural because there can be more than one per transaction.

...but after exploring several transactions on xmrchain, I noticed that the transaction public key tends to be repeated inside extra despite already being indicated in its dedicated field.

It's not repeated, it's only in the tx extra field. The fact a block explorer displays it in multiple places is down to that implementation.