在内部联接查询PHP + MySQL上查找具有相同名称的用户

I'm working on a search system very outdated in PHP+MySQL, now... The owner asked me to introduce two buttons to apply filters to the main query. First button should find only users with same first_name and last_name, the second one should find only users with same last_name. Now ATM the query uses different joins (inner and left do get other informations from other tables)

$query = "
  SELECT s1.*,s2.first_name,s2.last_name,s2.email,s2.id as subscriber_id,s2.phone,s2.info,a.id as agency_id,a.info as agency_info,a.invoice as agency_invoice
  FROM subscription as s1
  LEFT JOIN agency as a
  ON s1.agency=a.id
  INNER JOIN subscriber as s2
  ON s1.subscriber=s2.id
  WHERE $matches AND s1.deleted=0
  ORDER BY s1.id DESC
  LIMIT $maxid,$limit
";

Everything works properbly but if I add clauses to group users by same full_name it returns only one result per fullname...

So if I have a table like this

+------------+-----------+
| first_name | last_name |
+------------+-----------+
| Foo        | Bar       |
| Foo        | Bar       |
| Bar        | Foo       |
| Bar        | Foo       |
| John       | Doe       |
+------------+-----------+

It returns

+------------+-----------+
| first_name | last_name |
+------------+-----------+
| Foo        | Bar       |
| Bar        | Foo       |
+------------+-----------+ 

Now... the clauses that I add into query are these (count and groupby)

$query = "
  SELECT s1.*,s2.first_name,s2.last_name,s2.email,s2.id as subscriber_id,s2.phone,s2.info,a.id as agency_id,a.info as agency_info,a.invoice as agency_invoice, Count(s2.*) AS Cnt
  FROM subscription as s1
  LEFT JOIN agency as a
  ON s1.agency=a.id
  INNER JOIN subscriber as s2
  ON s1.subscriber=s2.id
  WHERE $matches AND s1.deleted=0
  GROUP BY s2.first_name, s2.last_name HAVING Cnt>1
  ORDER BY s1.id DESC
  LIMIT $maxid,$limit
";

But as I've already said... it doesn't work and I don't know why... where am I wrong? Does it depend from joining? Is there a way to find a solution or with that query I need to find another solution? Please help me, I've tried other solutions but no way! :D