diff -r 6c4ab6f0a6fc -r 0265ccdb9e6f src/HOL/Tools/Sledgehammer/sledgehammer_isar.ML
--- a/src/HOL/Tools/Sledgehammer/sledgehammer_isar.ML	Fri Aug 01 23:58:42 2014 +0200
+++ b/src/HOL/Tools/Sledgehammer/sledgehammer_isar.ML	Sat Aug 02 00:15:08 2014 +0200
@@ -93,28 +93,34 @@
       map (replace_dependencies_in_line (name, [])) lines
   | add_line_pass1 line lines = line :: lines
 
-fun add_lines_pass2 res _ [] = rev res
-  | add_lines_pass2 res (prev as (prev_name, prev_norm_t))
-      ((line as (name, role, t, rule, deps)) :: lines) =
+fun normalize role t =
+  t |> role = Conjecture ? (HOLogic.dest_Trueprop #> s_not #> HOLogic.mk_Trueprop)
+
+fun add_lines_pass2 res [] = rev res
+  | add_lines_pass2 res ((line as (name, role, t, rule, deps)) :: lines) =
     let
-      val norm_t = t |> role = Conjecture ? (HOLogic.dest_Trueprop #> s_not #> HOLogic.mk_Trueprop)
+      val norm_t = normalize role t
+      val is_duplicate =
+        exists (fn (prev_name, prev_role, prev_t, _, _) =>
+            member (op =) deps prev_name andalso
+            Term.aconv_untyped (normalize prev_role prev_t, norm_t))
+          res
 
       fun looks_boring () =
-        t aconv @{prop True} orelse t aconv @{prop False} orelse norm_t aconv prev_norm_t orelse
-        length deps < 2
+        t aconv @{prop True} orelse t aconv @{prop False} orelse length deps < 2
 
       fun is_skolemizing_line (_, _, _, rule', deps') =
         is_skolemize_rule rule' andalso member (op =) deps' name
 
       fun is_before_skolemize_rule () = exists is_skolemizing_line lines
     in
-      if (Term.aconv_untyped (prev_norm_t, norm_t) andalso member (op =) deps prev_name) orelse
+      if is_duplicate orelse
           (role = Plain andalso not (is_skolemize_rule rule) andalso
            not (is_arith_rule rule) andalso not (is_datatype_rule rule) andalso
            not (null lines) andalso looks_boring () andalso not (is_before_skolemize_rule ())) then
-        add_lines_pass2 res prev (map (replace_dependencies_in_line (name, deps)) lines)
+        add_lines_pass2 res (map (replace_dependencies_in_line (name, deps)) lines)
       else
-        add_lines_pass2 (line :: res) (name, norm_t) lines
+        add_lines_pass2 (line :: res) lines
     end
 
 type isar_params =
@@ -165,7 +171,7 @@
 
         val atp_proof =
           fold_rev add_line_pass1 atp_proof []
-          |> add_lines_pass2 [] (("", []), Term.dummy)
+          |> add_lines_pass2 []
 
         val conjs =
           map_filter (fn (name, role, _, _, _) =>