diff --git a/eval.c b/eval.c
index 6d6fccca..5db590c1 100644
--- a/eval.c
+++ b/eval.c
@@ -806,6 +806,12 @@ static sexp analyze_lambda (sexp ctx, sexp x, int depth) {
       sexp_return(res, sexp_compile_error(ctx, "non-symbol parameter", x));
     else if (sexp_truep(sexp_memq(ctx, sexp_car(ls), sexp_cdr(ls))))
       sexp_return(res, sexp_compile_error(ctx, "duplicate parameter", x));
+  if (! sexp_nullp(ls)) {
+    if (! sexp_idp(ls))
+      sexp_return(res, sexp_compile_error(ctx, "non-symbol parameter", x));
+    else if (sexp_truep(sexp_memq(ctx, ls, sexp_cadr(x))))
+      sexp_return(res, sexp_compile_error(ctx, "duplicate parameter", x));
+  }
   /* build lambda and analyze body */
   res = sexp_make_lambda(ctx, tmp=sexp_copy_list(ctx, sexp_cadr(x)));
   if (sexp_exceptionp(res)) sexp_return(res, res);
diff --git a/sexp.c b/sexp.c
index 1503e5b2..94d1d096 100644
--- a/sexp.c
+++ b/sexp.c
@@ -439,6 +439,7 @@ void sexp_init_context_globals (sexp ctx) {
   const char** features;
   int i, endianess_check = 1;
   sexp type, *vec, print=NULL;
+  sexp_gc_var1(feature);
   sexp_context_globals(ctx)
     = sexp_make_vector(ctx, sexp_make_fixnum(SEXP_G_NUM_GLOBALS), SEXP_VOID);
 #if ! SEXP_USE_GLOBAL_SYMBOLS
@@ -474,10 +475,15 @@ void sexp_init_context_globals (sexp ctx) {
   sexp_global(ctx, SEXP_G_FEATURES) = SEXP_NULL;
   sexp_push(ctx, sexp_global(ctx, SEXP_G_FEATURES), SEXP_FALSE);
   sexp_car(sexp_global(ctx, SEXP_G_FEATURES)) = sexp_intern(ctx, (*(unsigned char*) &endianess_check) ? "little-endian" : "big-endian", -1);
+  sexp_gc_preserve1(ctx, feature);
   for (features=sexp_initial_features; *features; features++) {
-    sexp_push(ctx, sexp_global(ctx, SEXP_G_FEATURES), SEXP_FALSE);
-    sexp_car(sexp_global(ctx, SEXP_G_FEATURES)) = sexp_intern(ctx, *features, -1);
+    feature = sexp_intern(ctx, *features, -1);
+    if (sexp_not(sexp_memq(ctx, feature, sexp_global(ctx, SEXP_G_FEATURES)))) {
+      sexp_push(ctx, sexp_global(ctx, SEXP_G_FEATURES), SEXP_FALSE);
+      sexp_car(sexp_global(ctx, SEXP_G_FEATURES)) = feature;
+    }
   }
+  sexp_gc_release1(ctx);
   sexp_global(ctx, SEXP_G_NUM_TYPES) = sexp_make_fixnum(SEXP_NUM_CORE_TYPES);
   sexp_global(ctx, SEXP_G_TYPES)
     = sexp_make_vector(ctx, sexp_make_fixnum(SEXP_INIT_NUM_TYPES), SEXP_VOID);