[Plugin: WordPress Form Manager] Unique Values for Form Entries

Hi, How can I make sure each entry is UNIQUE for each form element?
So that duplicate entries are not submitted?

FOr example if somebody had entered their email once in a form and submits another form with the same email, how can i display an email saying, this email already exists?

Thank you for your kind advice!

http://ww.wp.xz.cn/extend/plugins/wordpress-form-manager/