One solution would be getting the files one by one using a for loop and process them.
Untested:
ECHO Copy session %date% %time%>%LOG%
FOR %%f in (%SORC%\*.csv) do (
SET fname=%%~nxf
IF NOT exist %DEST%\%fname% (
xcopy %%f %SORC%\archive /q
move /Y %%f %DEST%
ECHO Copied file %fname%>>%LOG%
) ELSE (
ECHO Skipped file %fname%>>%LOG%
)
)