assembly code

while doing assembly code in Keil i got an error as
C:\KEIL\C51\INC\ATMEL\REGX51.H(178): error A10: ATTEMPT TO DEFINE AN ALREADY DEFINED SYMBOL
if i donot include the regx51 file the error is gone and i am able to build the code. why this error is coming. Eventhough in the regx51 file only the addresses of the SFR registers are mentioned still they are mentioning about the error..can you please give an explanation to this

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