Append running ALPHA to duplicating entries from flat file

hi all,

i have a requirement to append a running alpha to the end of a value from flat file should the value is a duplicating one. example:

flat file output

12345 12345

12346 12346

12345 12345A

12347 12347

12348 12348

12345 12345B

12345 12345C

12347 12347A

can i know the best approach to do the above?

thanks