L6002E: Symbol * multiply defined (by *.o and *.o)

dear arm,

 We have a problem when compiling with AC6.

 

[link] Error: L6200E: Symbol __assert multiply defined (by assert.o and retarget.o).
 

But the assert.c doesn't exist, we didn't create a c source file named assert. Also, we searched assert.o in map file, but didn't find it either. Why did this error occur?