Reading Reveive Hold Register from USART in SPI Mode on Arduino Due

Question

Sending with this function here works fine:

uint16_t usart0spiTransfer16(uint16_t spiMessage16) {
    uint16_t sdo = 0;
    byte firstByte = spiMessage16;
    byte secondByte = (spiMessage16 >> 8);
    while (((USART0->US_CSR  >> 1) & 1) == 0);
    digitalWrite(SS, LOW);                  
    USART0->US_THR = secondByte;                //write to USARTO Transmit Hold Register

    //this byte is returned twice
    secondByte = USART0->US_RHR;                //read from USART0 Receive Hold Register
    while (((USART0->US_CSR  >> 1) & 1) == 0);  //TXRDY: Transmitter Ready
    USART0->US_THR = firstByte;

    //trying to read a second time gives me the same value as before
    firstByte = USART0->US_RHR;

    while (((USART0->US_CSR  >> 9) & 1) == 0);
    digitalWrite(SS, HIGH);

    sdo = (firstByte << 8 ) | (secondByte & 0xFF);
    return sdo;
}

and when i wait for RXRDY after secondByte = USART0->US_RHR; with while (((USART0->US_CSR >> 0) & 1) == 0); i will stay there forever since it does not change

how do i read the registers value for the second time?