The 29-year-old Tyco Suzuki rider had not sprayed the champagne on the top step of the podium since round two at Thruxton but he was completely dominant at Donington and started on pole.
“It is pretty special to take a winning double in this championship,” said Brookes. “From the first session here the bike was perfect and I had a good feeling throughout.
"Now I am looking forward to getting to Assen and getting to the part that really counts.”
Despite a long delay due to a heavy oil spill in race one, Brookes - who was leading when the red flags were deployed - won a seven lap-dash with ease and repeated the trick in race two.
That was contested over the full 20 laps this time but there was to be no stopping Brookes as he once again held off the charge of Swan Yamaha's Tommy Hill and Michael Laverty of Samsung Honda.
Unlike race one, Hill could not hold off Laverty and they changed positions in the top three while another rider in the Showdown, the injured Shane Byrne, watched on elsewhere.
Tommy Bridewell, who had his place in the Showdown secured after race one, was just behind the top three in fourth while Alex Lowes did what was required of him to finish fifth.
That fifth-place finished confirmed Lowes as the sixth and final rider in the Showdown – a three-round battle for the title, which begins in two weeks' time at Assen in the Netherlands.
The equalisation procedure, designed to ensure a tense championship fight, puts Hill top with 535 points, Brookes second with 525 then Byrne (523), Laverty (514), Lowes (502) and Bridewell (500).
“Two podiums were what we needed today and it is nice to have a bit of a lead in the standings going in to the Showdown at Assen," said Hill.
"You can’t pay too much attention to it though as anything can happen. Cadwell Park was my weekend to do the double and this weekend it was Josh Brookes.
"It has been a bit frustrating for us as we wanted to try and come away with two double wins but we still have that 10 points over Josh so hopefully we can keep pushing on and come out on top at Assen."
1. Josh Brookes (Tyco Suzuki)
2. Michael Laverty (Samsung Honda) +0.436s
3. Tommy Hill (Swan Yamaha) +10.302s
4. Tommy Bridewell (Supersonic BMW) +14.026s
5. Alex Lowes (WFR Honda) +14.508s
6. James Westmoreland (WFR Honda) +15.361s
7. Stuart Easton (Rapid Solicitors Kawasaki) +15.797s
8. Luca Scassa (Padgetts Honda) +18.352s
9. Jon Kirkham (Samsung Honda) +18.371s
10. Chris Walker (Pr1mo Bournemouth Kawasaki) +19.485s