dic = [(1,"aa"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg"),(1,"bb")]

grp  = [(1,["aa","bb","cc"]), (2, ["aa"]), (3, ["ff","gg"])]

import Data.Function (on)
import Data.List (sortBy, groupBy)
import Data.Ord (comparing)

myGroup :: (Eq a, Ord a) => [(a, b)] -> [(a, [b])]
myGroup = map (l -> (fst . head $ l, map snd l)) . groupBy ((==) `on` fst)
          . sortBy (comparing fst)

[(1,"aa"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg"),(1,"bb")]
=> [(1,"aa"),(1,"bb"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg")]

[(1,"aa"),(1,"bb"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg")]
=> [[(1,"aa"),(1,"bb"),(1,"cc")],[(2,"aa")],[(3,"ff"),(3,"gg")]]

[[(1,"aa"),(1,"bb"),(1,"cc")],[(2,"aa")],[(3,"ff"),(3,"gg")]]
=> [(1,["aa","bb","cc"]), (2, ["aa"]), (3, ["ff","gg"])]`)

> myGroup dic
[(1,["aa","bb","cc"]),(2,["aa"]),(3,["ff","gg"])]

import Data.Map
sortAndGroup assocs = fromListWith (++) [(k, [v]) | (k, v) <- assocs]

*Main> sortAndGroup [(1,"aa"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg"),(1,"bb")]
fromList [(1,["bb","cc","aa"]),(2,["aa"]),(3,["gg","ff"])]

Prelude> :set -XTransformListComp
Prelude> import GHC.Exts (groupWith, the)
Prelude GHC.Exts> let dic = [ (1, "aa"), (1, "bb"), (1, "cc") , (2, "aa"), (3, "ff"), (3, "gg")]
Prelude GHC.Exts> [(the key, value) | (key, value) <- dic, then group by key using groupWith]
[(1,["aa","bb","cc"]),(2,["aa"]),(3,["ff","gg"])]

grp :: Eq a => [(a,b)] -> [(a,[b])]
  grp = foldr f []
     where
       f (z,s) [] = [(z,[s])]
       f (z,s) a@((x,y):xs)  | x == z = (x,s:y):xs
                             | otherwise = (z,[s]):a

grp [] = []
grp ((a, b): xs) = (a, b : map snd forthis) : grp rest where
  (forthis, rest) = partition ((== a) . fst) xs