Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Silicon Labs/Series1/EFR32ZG14P/EFR32ZG14P231F256GM32/USART0/RXDOUBLE#0x0
RX FIFO Double Data Register
RX Data 0
RX Data 1
https://github.com/cmsis-svd/cmsis-svd-data