Does BOFC allows to create duplicate fields or displays any error if fields with same API name are created?

Yes, Bulk Create Field feature is smart enough to identify the fields with the same api name.

Experience it for yourself

Bulk Object Field Creator (BOFC) is a must-have tool, designed specifically for Salesforce.com Administrators, that simplifies many of the tedious tasks of managing a complex Salesforce environment.

This tool is available on Appexchange and Click on below button to install the
latest version of BOFC

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