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?