quotes for destination for me was needed cause of being a network drive tested and works awesomely

@echo off
set Source=C:\wgwcc.zip
set Destination="\\ajp\tonybook\nightly\"
set Filename=wgwcc
set a=1
:loop
if exist %Destination%\%Filename%(%a%).zip set /a a+=1 && goto :loop
copy %Source% %Destination%\%Filename%(%a%).zip