fixup forbid duplicate parameter names

Author: leonerd

op.c

@@ -16990,8 +16990,21 @@ Perl_subsignature_append_named(pTHX_ const char *paramname, PADOFFSET padix, OPC
         return;
     }
 
+    SV *namesv = newSVpvn_utf8(paramname, strlen(paramname), true);
+
+    for(size_t parami = 0; parami < signature->named_params; parami++) {
+        struct yy_parser_signature_param *param = signature->params + parami;
+
+        if(!param->paramname || !sv_streq(param->paramname, namesv))
+            continue;
+
+        SvREFCNT_dec(namesv);
+        yyerror("Duplicated subroutine parameter name");
+        return;
+    }
+
     struct yy_parser_signature_param *param = subsignature_push_param();
-    param->paramname = newSVpvn_utf8(paramname, strlen(paramname), true);
+    param->paramname = namesv;
 
     signature->named_params++;
 

pod/perldiag.pod

@@ -2356,6 +2356,12 @@ already been freed.
 (W unpack) You have applied the same modifier more than once after a
 type in a pack template.  See L<perlfunc/pack>.
 
+=item Duplicated subroutine parameter name
+
+(F) You have attempted to use a subroutine parameter name a second time.
+
+    sub f ( :$name, :$name ) { ... }
+
 =item each on anonymous %s will always start from the beginning
 
 (W syntax) You called L<each|perlfunc/each> on an anonymous hash or

t/lib/croak/signatures

@@ -81,6 +81,12 @@ sub t060 (@a, $b = 222) { }
 EXPECT
 Slurpy parameter not last at - line 2, near "222) "
 ########
+# NAME duplicated named parameter
+no warnings 'experimental::signature_named_parameters';
+sub f (:$x, :$x) { }
+EXPECT
+Duplicated subroutine parameter name at - line 3, near ":$x) "
+########
 sub t061 (@a, @b) { }
 EXPECT
 Multiple slurpy parameters not allowed at - line 2, near "@b) "