diff --git a/include/flatbuffers/flexbuffers.h b/include/flatbuffers/flexbuffers.h
index a45d14b12..9270f3e69 100644
--- a/include/flatbuffers/flexbuffers.h
+++ b/include/flatbuffers/flexbuffers.h
@@ -900,6 +900,7 @@ class Builder FLATBUFFERS_FINAL_CLASS {
           BuilderFlag flags = BUILDER_FLAG_SHARE_KEYS)
       : buf_(initial_size),
         finished_(false),
+        has_duplicate_keys_(false),
         flags_(flags),
         force_min_bit_width_(BIT_WIDTH_8),
         key_pool(KeyOffsetCompare(buf_)),
@@ -1124,12 +1125,16 @@ class Builder FLATBUFFERS_FINAL_CLASS {
                 auto bs = reinterpret_cast<const char *>(
                     flatbuffers::vector_data(buf_) + b.key.u_);
                 auto comp = strcmp(as, bs);
-                // If this assertion hits, you've added two keys with the same
-                // value to this map.
+                // We want to disallow duplicate keys, since this results in a
+                // map where values cannot be found.
+                // But we can't assert here (since we don't want to fail on
+                // random JSON input) or have an error mechanism.
+                // Instead, we set has_duplicate_keys_ in the builder to
+                // signal this.
                 // TODO: Have to check for pointer equality, as some sort
                 // implementation apparently call this function with the same
                 // element?? Why?
-                FLATBUFFERS_ASSERT(comp || &a == &b);
+                if (!comp && &a != &b) has_duplicate_keys_ = true;
                 return comp < 0;
               });
     // First create a vector out of all keys.
@@ -1143,6 +1148,10 @@ class Builder FLATBUFFERS_FINAL_CLASS {
     return static_cast<size_t>(vec.u_);
   }
 
+  // Call this after EndMap to see if the map had any duplicate keys.
+  // Any map with such keys won't be able to retrieve all values.
+  bool HasDuplicateKeys() const { return has_duplicate_keys_; }
+
   template<typename F> size_t Vector(F f) {
     auto start = StartVector();
     f();
@@ -1574,6 +1583,7 @@ class Builder FLATBUFFERS_FINAL_CLASS {
   std::vector<Value> stack_;
 
   bool finished_;
+  bool has_duplicate_keys_;
 
   BuilderFlag flags_;
 
diff --git a/src/idl_parser.cpp b/src/idl_parser.cpp
index 3f0407734..9800501dc 100644
--- a/src/idl_parser.cpp
+++ b/src/idl_parser.cpp
@@ -2857,6 +2857,8 @@ CheckedError Parser::ParseFlexBufferValue(flexbuffers::Builder *builder) {
                                });
       ECHECK(err);
       builder->EndMap(start);
+      if (builder->HasDuplicateKeys())
+        return Error("FlexBuffers map has duplicate keys");
       break;
     }
     case '[': {