CREATE TEMPORARY TABLE tt1
SELECT DISTINCT t1.string AS IP_Adr, t2.mac_address AS MAC
FROM ip_addresses t1, devices t2, activity t3
WHERE t3.device_id = t2.id
AND t3.ip_address_id = t1.id
AND t2.ip_static_id IS NOT NULL
ORDER BY t1.string;

CREATE TEMPORARY TABLE tt2
SELECT *, COUNT(IP_Adr) AS anz FROM tt1
GROUP BY IP_Adr;

SELECT a.IP_Adr, a.MAC FROM tt1 a, tt2 bA
WHERE a.IP_Adr = b.IP_Adr
AND b.anz >1