count(*) to avoid duplicates

[datadriven]

This is for (2) seperate sites that share the same login table (same username/passwords will access both sites).

Users may register at either site, if they choose, then they should login at the other site to "complete" the last phase of registration at the other site to insert remaining needed values.

Problem is with duplicates. When some users won't login, as they should, "complete" the process at the other site. They may bypass and register anew like anyone else at the site for the first time. (Even though there is a pre-registration page at both sites -- asking if users have already registered at the other site prior to arriving at the second -- directing a login to complete the process.)

Reason this is a concern is that one site has a (6) page registration process, inserting to (8) tables.

My idea (aside from aleady checking for unique inserts on usernames/passwords/emails) is to check on the complete phone number.
To be used on a conditional show region on the second page of registration -- if the latest registrant entered the same phone number asmay aleady exist in the table, (previously enterered while registering on the other site and forgot about it) -- this 2nd registration page will not show due to the: count(*)

Starting with 'Area' code, is this practical/how to combine all three?
Anyone have better suggestions or experience on handling two site duiplicates?

Would there be any benefit in having ALL USERS (both new and prior from the "other" site) fill out the the 1st registration form (on the (6) page registration site) where they are then directed to login to complete the process?

'AREA' 'PRE' 'PHONE'

SELECT *
FROM `Members`
WHERE `Area`
IN (
SELECT `Area`
FROM 'Members`
GROUP BY `Area`
HAVING count(*)>1
)

note: some registrants have same company names fir satellite/regional offices