Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Silicon Labs/Series1/MGM13/MGM13S12F512GN/USART0/RXDOUBLE#0x0
RX FIFO Double Data Register
RX Data 0
RX Data 1
https://github.com/cmsis-svd/cmsis-svd-data