How can I make multiple joins on the same table?
I think this does what you need

 SELECT s.Id, eeav.Name, IFNULL(ee.Name, ee2.Name) AS 'Name' FROM signal s
 LEFT JOIN equipmententityactualvalue eeav ON (s.Eeav = eeav.Id)
 LEFT JOIN equipmententity ee ON (eeav.Ee = ee.Id)
 LEFT JOIN equipmententity ee2 ON (s.Ee = ee2.Id)