Count rows from a group by table with filter

I hope you can help me:

I have two tables: a dimension and a fact table for users.

I would like to find how many users have registered twice based on the VAT number.

So I want to do a group by VAT number, filter rows that have a count(distinct id)>1 and then count how many rows do I have.

Furthermore there is one more filter I should consider: since it is a dimension table, I must take only the last snap.

So

1. Get last snap from fact_users (is_last_snap = true)
2. Get rows with duplicates (count(distinct id)>1)
3. Count how many rows do I have

So, my output is supposed to be a simple number.

I know how to do a GROUP BY table with DAX but I don't know how to put filter in it.

GroupedByCF = 
GROUPBY(
 dim_users,
 dim_users[VAT],
 "RowCount",
 COUNTX(
 CURRENTGROUP(), 
dim_users[id]
 )
)

This could potentially work (I can also put filter "RowCount>1" into the next measure but I don't know how to put the last snap (since it comes from another table). 

So my Idea was to create a table like this and then to create a measure to count rows.

If it was a SQL I would do something like this

WITH CTE AS (
    SELECT d.vat as vat,
           count(distinct d.id) as counting
    FROM fact.users f
    LEFT JOIN dim.users d ON f.id_tk = d._tk
    WHERE 1 = 1
    AND f.d_date_tk = get_last_msnap_date()
    GROUP BY d.vat
    HAVING count(distinct d.id) > 1
)
SELECT COUNT(*)
FROM CTE