Davies won Sunday's second race ahead of runner-up Rea of Northern Ireland, with England's Tom Sykes in third.
It was the same 1-2-3 in Saturday's first race at the Italian circuit.
Rea, who has been on the podium in all 10 races so far, heads the WSB standings on 221, but Davies has narrowed the gap to 35 points.
Huddersfield's Sykes is third on 154 points in the championship which now heads to Sepang in Malaysia for round six.
It was a second double of the year for Ducati rider Davies as he won both races of the third round at Aragon.
The 29-year-old led from start to finish to come home 4.262 seconds ahead of Rea.
Welshman Chaz Davies completed an impressive double at Imola to underline his position as the main challenger to World Superbikes champion Jonathan Rea.