make sure completions are unique

See JuliaLang/julia#28694

Author: stevengj

src/handlers.jl

@@ -146,7 +146,7 @@ function complete_request(socket, msg)
         comps_, positions = REPLCompletions.completions(code[codestart:end], cursorpos-codestart+1)
     end
     @static if isdefined(REPLCompletions, :completion_text)
-        comps = REPLCompletions.completion_text.(comps_) # julia#26930
+        comps = unique!(REPLCompletions.completion_text.(comps_)) # julia#26930
     else
         comps = comps_
     end