ieduplicates: warning if filepath for duplicates report has no extension

[luizaandrade]

as discussed in #94, we should check if either .xls or .xslx are specified, and show a warning saying to double check the filepath if the extension is not specified

[kbjarkefur]

Do not implement directly. Instead, make sure this feature exists in utility sub-programs in #146, and then use that sub-program here