Preventing duplicate entries by respondents

Hi Kal_Lam,
Could you please make more clear about “To make this work, you must link the project to itself”. I enable “Data sharing enabled”. then in the same form, I imported itself (select phone number that I want to validate the duplicate), but it do not work. thanks
My register form

my setting of this form in kobo

my form

when I try to enter the same phone number, Kobo could not find the duplicate.

Please help