define appropriate function: stringDuplicates[string_String]:=Select[Counts[Characters[string]],#>1&] In[2]:= Select[Counts[Characters["aquickbrownfox"]],#>1&] Out[2]= 2|> In[5]:= stringDuplicates["aquickbrownandredfox"] Out[5]= 2,r->2,o->2,n->2,d->2|>