Have a look at his easily configurable module: https://apps.odoo.com/apps/modules/9.0/crm_duplicates/

No, there is not any custom module that will ask for for confirmation before creating a new partner which has the same phone or fax as another partner.

You have to make custom module as per your expectation. please have a look use ofConstraints(http://stackoverflow.com/questions/13263507/openerp-unique-constraint) and sql_constraints

This will fix your problem.

You can prevent the creation of 2 contacts with the same phone number by just making that field "unique" in the settings. This applies to any field.

In case someone desperately searches it, just as I did: The duplicate search and merge feature is weirdly hidden in Odoo 10, yet another item contributing to Odoo's usability catastrophy.

In a list view of contacts, select two random contacts, then hit "Action" > "Merge Selected Contacts". Hit "Skip these contacts". Hit "Deduplicate the other contacts". This will prompt you with the duplicate search form.

Again, unbelievable...

I found this : The Heo_Unique_Partner_Ref module can prevent the creation of two partners with the same reference.

Is a Heonium module (http://www.heonium.com), you can see video presentation (https://www.youtube.com/watch?v=v4XVcUEJr5c)

This module adds a reference field below the name of the customer (in blue, so required)

I am new to the forums, so I can't upvote or comment on specific answers yet, but I wanted to comment that the solution provided by Nikolai Ensslen worked for me:

"In a list view of contacts, select two random contacts, then hit "Action" > "Merge Selected Contacts". Hit "Skip these contacts". Hit "Deduplicate the other contacts". This will prompt you with the duplicate search form."

This is still working: 

"In a list view of contacts, select two random contacts, then hit "Action" > "Merge Selected Contacts". Hit "Skip these contacts". Hit "Deduplicate the other contacts". This will prompt you with the duplicate search form.

Still no other way to access this crucial function! (??)