The /c switch takes care of that. The only file that I know fails to copy every time is the swap file. Trust me, I used to use this method all the time. You can review what the other switches do here:

XCOPY source [destination] [/A | /M] [/D[:date]] [/P] [/S [/E]] [/V] [/W]

                           [/C]  [/Q] [/F] [/L] [/G] [/H] [/R] [/T] [/U]

                           [/K] [/N] [/O] [/X] [/Y] [/-Y] [/Z]

                           [/EXCLUDE:file1[+file2][+file3]...]



  source       Specifies the file(s) to copy.

  destination  Specifies the location and/or name of new files.

  /A           Copies only files with the archive attribute set,

               doesn't change the attribute.

  /M           Copies only files with the archive attribute set,

               turns off the archive attribute.

  /D:m-d-y     Copies files changed on or after the specified date.

               If no date is given, copies only those files whose

               source time is newer than the destination time.

  /EXCLUDE:file1[+file2][+file3]...

               Specifies a list of files containing strings.  Each string

               should be in a separate line in the files.  When any of the

               strings match any part of the absolute path of the file to be

               copied, that file will be excluded from being copied.  For

               example, specifying a string like \obj\ or .obj will exclude

               all files underneath the directory obj or all files with the

               .obj extension respectively.

  /P           Prompts you before creating each destination file.

  /S           Copies directories and subdirectories except empty ones.

  /E           Copies directories and subdirectories, including empty ones.

               Same as /S /E. May be used to modify /T.

  /V           Verifies each new file.

  /W           Prompts you to press a key before copying.

  /C           Continues copying even if errors occur.

  /I           If destination does not exist and copying more than one file,

               assumes that destination must be a directory.

  /Q           Does not display file names while copying.

  /F           Displays full source and destination file names while copying.

  /L           Displays files that would be copied.

  /G           Allows the copying of encrypted files to destination that does

               not support encryption.

  /H           Copies hidden and system files also.

  /R           Overwrites read-only files.

  /T           Creates directory structure, but does not copy files. Does not

               include empty directories or subdirectories. /T /E includes

               empty directories and subdirectories.

  /U           Copies only files that already exist in destination.

  /K           Copies attributes. Normal Xcopy will reset read-only attributes.

  /N           Copies using the generated short names.

  /O           Copies file ownership and ACL information.

  /X           Copies file audit settings (implies /O).

  /Y           Suppresses prompting to confirm you want to overwrite an

               existing destination file.

  /-Y          Causes prompting to confirm you want to overwrite an

               existing destination file.

  /Z           Copies networked files in restartable mode.



The switch /Y may be preset in the COPYCMD environment variable.

This may be overridden with /-Y on the command line.