But I do not understand the problem. When doing:
<verbatim>
  PS notepad camelCase.txt
  PS fossil add camelCase.txt
  ADDED  camelCase.txt
  PS fossil add camelcase.txt
  ADDED  camelcase.txt
</verbatim>
Command "fossil add ..." needs to ask the operating system if file 'camelcase.txt' exist. If it does not exist, the command fails. When doing this request, fossil can ask the operating system for the exact name of the file. In this case, the OS would answer "camelCase.txt". Then, it would be easy to detect that "camelCase.txt" has already been added and raise an error.