Overwrite weak Systick_handler function

Hi all,

I want to overwrite the weak systick_handler function for a stm32f745 chip - it is labeled as weak in the Src\startup_stm32f745xx.s(258) :                 EXPORT  SysTick_Handler            [WEAK] 

startup file, but when I try to overwrite it anywhere else 

(using the extern "C" configuration)

I get this: Error: L6200E: Symbol SysTick_Handler multiply defined (by irq_cm4f.o and gpio_base.o)

NOTE: I cannot edit the startup file, it is locked to my project. 

Am I missing something else?