diff --git a/compiler/common.m b/compiler/common.m
index e9b8fc341..1c9446c1a 100644
--- a/compiler/common.m
+++ b/compiler/common.m
@@ -1,5 +1,5 @@
 %---------------------------------------------------------------------------%
-% Copyright (C) 1995-2002 The University of Melbourne.
+% Copyright (C) 1995-2003 The University of Melbourne.
 % This file may only be copied under the terms of the GNU General
 % Public License - see the file COPYING in the Mercury distribution.
 %---------------------------------------------------------------------------%
@@ -155,12 +155,25 @@ common__optimise_unification(Unification0, _Left0, _Right0, Mode, _Context,
 				construction, Info0, OldStruct)
 		->
 			OldStruct = structure(OldVar, _, _, _),
-			UniMode = ((free - Inst) -> (Inst - Inst)),
-			common__generate_assign(Var, OldVar, UniMode,
-				GoalInfo0, Goal - GoalInfo, Info0, Info1),
-			simplify_info_set_requantify(Info1, Info2),
-			pd_cost__goal(Goal0 - GoalInfo0, Cost),
-			simplify_info_incr_cost_delta(Info2, Cost, Info)
+			( ArgVars = [] ->
+				% Constants don't use memory, so there's
+				% no point optimizing away their
+				% construction -- in fact, doing so
+				% could cause more stack usage.
+				common__record_equivalence(Var, OldVar,
+					Info0, Info),
+				Goal = Goal0,
+				GoalInfo = GoalInfo0
+			;
+				UniMode = ((free - Inst) -> (Inst - Inst)),
+				common__generate_assign(Var, OldVar, UniMode,
+					GoalInfo0, Goal - GoalInfo,
+					Info0, Info1),
+				simplify_info_set_requantify(Info1, Info2),
+				pd_cost__goal(Goal0 - GoalInfo0, Cost),
+				simplify_info_incr_cost_delta(Info2,
+					Cost, Info)
+			)
 		;
 			Goal = Goal0,
 			GoalInfo = GoalInfo0,
@@ -356,22 +369,15 @@ common__vars_are_equiv(X, Y, VarEqv) :-
 common__record_cell(Var, ConsId, ArgVars, Info0, Info) :-
 	simplify_info_get_common_info(Info0, CommonInfo0),
 	simplify_info_get_var_types(Info0, VarTypes),
-	( ArgVars = [] ->
-		% Constants do not have memory cells to reuse,
-		% at least in the memory models we are interested in.
-		CommonInfo = CommonInfo0
-	;
-		CommonInfo0 = common(VarEqv, StructMapAll0,
+	CommonInfo0 = common(VarEqv, StructMapAll0,
 			StructMapLastCall0, SeenCalls),
-		map__lookup(VarTypes, Var, VarType),
-		Struct = structure(Var, VarType, ConsId, ArgVars),
-		common__do_record_cell(StructMapAll0, ConsId,
-			Struct, StructMapAll),
-		common__do_record_cell(StructMapLastCall0, ConsId, Struct,
+	map__lookup(VarTypes, Var, VarType),
+	Struct = structure(Var, VarType, ConsId, ArgVars),
+	common__do_record_cell(StructMapAll0, ConsId, Struct, StructMapAll),
+	common__do_record_cell(StructMapLastCall0, ConsId, Struct,
 			StructMapLastCall),
-		CommonInfo = common(VarEqv, StructMapAll,
-			StructMapLastCall, SeenCalls)
-	),
+	CommonInfo = common(VarEqv, StructMapAll,
+			StructMapLastCall, SeenCalls),
 	simplify_info_set_common_info(Info0, CommonInfo, Info).
 
 :- pred common__do_record_cell(struct_map, cons_id, structure, struct_map).
diff --git a/tests/warnings/Mercury.options b/tests/warnings/Mercury.options
index af5dbc8cd..15b1c9806 100644
--- a/tests/warnings/Mercury.options
+++ b/tests/warnings/Mercury.options
@@ -12,6 +12,7 @@ MCFLAGS-arg_order_rearrangment	= --introduce-accumulators \
 					--trace-optimized
 
 MCFLAGS-duplicate_call		= --warn-duplicate-calls
+MCFLAGS-duplicate_const		= --warn-duplicate-calls
 MCFLAGS-unused_args_analysis    = --intermodule-analysis \
 				--optimize-unused-args --warn-unused-args
 MCFLAGS-unused_args_analysis2   = --intermodule-analysis \
diff --git a/tests/warnings/Mmakefile b/tests/warnings/Mmakefile
index 53c062d14..6557d1a48 100644
--- a/tests/warnings/Mmakefile
+++ b/tests/warnings/Mmakefile
@@ -12,6 +12,7 @@ ERRORCHECK_PROGS= \
 	det_infer_warning \
 	double_underscore \
 	duplicate_call \
+	duplicate_const \
 	inf_recursion_lambda \
 	infinite_recursion \
 	inference_test \
diff --git a/tests/warnings/duplicate_const.exp b/tests/warnings/duplicate_const.exp
new file mode 100644
index 000000000..1eef10612
--- /dev/null
+++ b/tests/warnings/duplicate_const.exp
@@ -0,0 +1,2 @@
+duplicate_const.m:016: Warning: redundant call to predicate `duplicate_const.called/4'.
+duplicate_const.m:015: Here is the previous call to predicate `duplicate_const.called/4'.
diff --git a/tests/warnings/duplicate_const.m b/tests/warnings/duplicate_const.m
new file mode 100644
index 000000000..14df20552
--- /dev/null
+++ b/tests/warnings/duplicate_const.m
@@ -0,0 +1,20 @@
+% Test the warning for duplicate calls where some of the arguments are
+% duplicated constants.
+:- module duplicate_const.
+
+:- interface.
+
+:- pred dup_call(int::in, int::in, int::out) is det.
+
+:- pred called(T::in, int::in, int::in, int::out) is det.
+
+:- implementation.
+:- import_module int.
+
+dup_call(Int1, Int2, Int) :-
+	called(1, Int1, Int2, Int3),
+	called(1, Int1, Int2, Int4),
+	Int is Int3 + Int4.
+
+called(_, Int1, Int2, Int) :-
+	Int is Int1 + Int2.