[dsp1234]

shouldn't you have "transactions", "transaction_senders", and "transaction_receivers"

were doing the right way(TM), that wouldn't happen

Having a separate transaction_senders and transaction_receivers wouldn't be the right way(TM) unless it's possible to have multiple senders/receivers. If a transaction can only have 1 sender/receiver then normalization is complete when the sender/receiver data is removed from 'transactions' table and replaced with the sender_id and receiver_id columns.

When introducing transaction_senders/transaction_receivers tables without further constraints, it would be immediately possible to have multiple transaction senders and multiple transaction receivers for a single transaction, which is likely an error.