Sometimes when opening a file, especially from an email message, the file
name is chnaged and is suffixed with a number in brackets.

I thought it could be to do with having multiple files of the same name open
but this is not the case, even if this is the opnly file open at the time I
seem to get the issue.

Please advise.

When you open an email attachment, the attached file is written to a
particular TEMP drive on your drive. The software must be trained to
rename a file with a number rather than overwrite an existing file.