..\OBJ\ADC.axf: Warning: L6304W: Duplicate input file ..\obj\system_stm32f10x_1.o ignored.

This warning message is indicating that the linker has detected that there are two copies of the same input file in the object directory. In this case, the file in question is "system_stm32f10x_1.o". The linker will ignore one of the duplicates to avoid any potential conflicts or errors during the linking process.

This warning message is not a critical error and can usually be safely ignored. However, it is always a good practice to check the build and make sure that all required files are included and linked correctly.