Removing duplicate rows and appending from a table of lists

You can use GroupBy and KeyValueMap:

x3  = KeyValueMap[Join[{#}, ## & @@ Reverse@SortBy[First][#2]] &]@
    GroupBy[x, First -> Rest]

Also

x4 = KeyValueMap[Join[{#}, #2] &]@
  GroupBy[x, First -> Rest, Flatten[Reverse@SortBy[First]@#, 1] &]

x2 == x3 == x4

True

rule=Merge[Cases[
  Reverse@x, {w_,{y__},{z__}} :> w-> {{y},{z}}],Catenate]

And:

(Join[{#},#/.rule]&/@DeleteDuplicates[x[[All,1]]])==x2

True

If there is no good reason why {3, 6, 10, 3, 6}, should appear before {3, 6, 9, 3, 6} in the merged list (x2), change Reverse@x to x, but perhaps I am missing something important here?

Original Post

rule_alt=Catenate/@Merge[Cases[
  Reverse@x, {w_,{y__},{z__}} :> w-> {{y},{z}}],Join]