Hi
@gadado
I suspect you prepared multiple transactions using the same feeSourceAddress, then tried to sign and broadcast them sequentially. Because the tx field output from register_prepare is the actual transaction exactly as it is broadcast on the network, it will spend a specific UTXO on that address as an input. Since you (probably) prepared all the registration transactions at the same time, they are trying to double spend the same UTXO for the transaction fee, which is not permitted by the network. Only the first transaction will succeed. You will need to run register_prepare again to use a different UTXO on the same address for your subsequent registration transactions.
Hope this makes sense, good luck!