warn if saving files with the same name

[wibeasley]

Sybil Andrieux at CHOP posted a suggestion that the redcap_file_download_oneshot() documentation page warn the user if they're trying to save multiple files with the same name. Even if overwrite is TRUE.

Post on private forums:
https://redcap.vanderbilt.edu/community/post.php?id=225360&comment=225418