Kelly couldn't have got any closer ; three seconds in the Tour of Flanders. Eddy Merckx won all five. Not sure if anyone else has done it. Hinault only won three of them. Hard to believe that Valverde has only won one monument but he has won Liege four times. Moser, Argentin and Saronni won three of the five.
Just look at Kellys program in 1985... it's crazy. From Criterium International on the 23rd of march he did De Panne, Harelbeke, Pais Vasco, Flanders, GW, Roubaix, FW and Liege. And two days after LBL he was at the start of La Vuelta.
But still one of the most memorable things in modern cycling, Kelly did a year prior: That year he was second in Sanremo. On march 24th and 25th he won Critérium International and all three stages. It was in Antibes back then. He then travelled to Belgium and started De Panne with one stage podium.
Then it gets really crazy. He then got second again in Flanders, only to fly to Spain the same day after a 270k monument and start Pais Vasco the next day and to WIN the first stage. Win another two stages, one being a time trial and the overall. Then travel back to Compiègne, start Roubaix two days after winning Pais Vasco and yes...winning that as well. And no we are not done here.
Back in the day there was no Schelde whatsoever or Brabantse Pijl leg stretching. Four days after Roubaix there was a full 250k Flèche Wallonne with the debut of the Mur du Huy. He got the 12th place or he used it to get Roubaix out of his body. Because on Sunday the guy went on to win LBL.
Only Eddy Merckx was able to win three monuments in one season. He did this four times. Sanremo, Flanders and LBL in 1969 and 1975. Sanremo, LBL and GDL in 1971 and 1972.
The double that is most likely to be achieved is of course RVV and PR. It was done twelve times with Cancellara and Boonen being the two riders, who did it twice.
Then it gets difficult when your name isn't cannibal. John Degenkolb was the only rider over a period from 1987 to 2015, who scored another double. He did this in 2015 by winning Sanremo and PR. Only Kelly and Cyrille van Hauwaert in 1908 did the same. Which is really interesting.
The second most common double is the italian one, which was - when we leave out Eddy - done by the italian cycling legends Giradengo, Binda, Mara, Bartali and Coppi. And it was last done in 1949. Actually in 2018 Vincenzo Nibali came really close. Only Thibaut Pinot stood between him and entering this hall of fame of italian cycling.
What also stands out to me, is that the most doable double beside the northern one, seems to be LBL and GDL. Only Merckx and Moreno Argentin went for it. Normally and specially in our day, there sould be a lot of potential candidates for that. Loulou, Roglic, Hirschi...