Creating unique keys from list

Clear[f]
f[_] = 0;
(f[#] = f[#] + 1; {f[#], #}) & /@ list

{{1, "b"}, {1, "a"}, {2, "b"}, {3, "b"}, {2, "a"}, {1, "c"}, {2, "c"}, {3, "c"}, {1, "d"}, {4, "b"}, {1, "aa"}}

list = {"b", "a", "b", "b", "a", "c", "c", "c", "d", "b", "aa"}
c = Association[# -> 0 & /@ Union@list]
{#, c[#] += 1} & /@ list

{{"b", 1}, {"a", 1}, {"b", 2}, {"b", 3}, {"a", 2}, {"c", 1}, {"c", 2}, {"c", 3}, {"d", 1}, {"b", 4}, {"aa", 1}}

Functional style:

FoldPairList[With[{c = Lookup[#1, #2, 1]}, 
   {{#2, c}, Append[#1, #2 -> c + 1]}] &, <||>, list]

Related