import kernel32,\
     GetSystemWow64Directory,'GetSystemWow64DirectoryA'
    

Error: symbol already defined

undefined symbol GetSystemWow64Directory

format PE GUI 4.0

entry start 

include 'win32ax.inc'
include 'api\user32.inc'
include 'api\kernel32.inc'

section '.data' data readable writeable

a rb MAX_PATH  

section '.code' code readable executable

start:
      invoke GetSystemWow64Directory, a, MAX_PATH
      cmp eax, 0
      je exit
 
      invoke MessageBox, 0, a, '', MB_OK

exit:
      invoke ExitProcess, 0        
      
section '.idata' import data readable

 library kernel32,'kernel32.dll',\
      user32,'user32.dll'

import kernel32,\
     GetSystemWow64Directory,'GetSystemWow64DirectoryA'
    

format PE GUI 4.0

entry start

include 'win32a.inc'

section '.data' data readable writeable

a rb MAX_PATH

section '.code' code readable executable

start:
      invoke GetSystemWow64Directory, a, MAX_PATH
      cmp eax, 0
      je exit

      invoke MessageBox, 0, a, '', MB_OK

exit:
      invoke ExitProcess, 0

section '.idata' import data readable

 library kernel32,'kernel32.dll',\
      user32,'user32.dll'

import kernel32,\
     ExitProcess,'ExitProcess',\
     GetSystemWow64Directory,'GetSystemWow64DirectoryA'

import user32,\
     MessageBox,'MessageBoxA'    

If you add custom APIs to kernel32 then you can't use the default fasm libraries.

Something like this, where you manually construct the API tables, and remove the 'x' from win32a.inc:

Code:

format PE GUI 4.0

entry start

include 'win32a.inc'

section '.data' data readable writeable

a rb MAX_PATH

section '.code' code readable executable

start:
      invoke GetSystemWow64Directory, a, MAX_PATH
      cmp eax, 0
      je exit

      invoke MessageBox, 0, a, '', MB_OK

exit:
      invoke ExitProcess, 0

section '.idata' import data readable

 library kernel32,'kernel32.dll',\
      user32,'user32.dll'

import kernel32,\
     ExitProcess,'ExitProcess',\
     GetSystemWow64Directory,'GetSystemWow64DirectoryA'

import user32,\
     MessageBox,'MessageBoxA'