Made "field set more than once" check in JSON parser faster.

Change-Id: I3ecc1aa610526c270faa56cc5266f14cd81db247
Tested: on Linux.
diff --git a/src/idl_parser.cpp b/src/idl_parser.cpp
index 4c60fd7..ed4476d 100644
--- a/src/idl_parser.cpp
+++ b/src/idl_parser.cpp
@@ -457,10 +457,6 @@
                             || struct_def.fields.vec[fieldn] != field)) {
        Error("struct field appearing out of order: " + name);
     }
-    for (auto it = field_stack_.rbegin();
-             it != field_stack_.rbegin() + fieldn; ++it) {
-      if (it->second == field) Error("field already set: " + name);
-    }
     Expect(':');
     Value val = field->value;
     ParseAnyValue(val, field);
@@ -469,6 +465,16 @@
     if (IsNext('}')) break;
     Expect(',');
   }
+  for (auto it = field_stack_.rbegin();
+           it != field_stack_.rbegin() + fieldn; ++it) {
+    if (it->second->used)
+      Error("field set more than once: " + it->second->name);
+    it->second->used = true;
+  }
+  for (auto it = field_stack_.rbegin();
+           it != field_stack_.rbegin() + fieldn; ++it) {
+    it->second->used = false;
+  }
   if (struct_def.fixed && fieldn != struct_def.fields.vec.size())
     Error("incomplete struct initialization: " + struct_def.name);
   auto start = struct_def.fixed