Add feature to error on duplicate key in Map<String, Value>

Cargo.toml

@@ -86,3 +86,8 @@ raw_value = []
 # overflow the stack after deserialization has completed, including, but not
 # limited to, Display and Debug and Drop impls.
 unbounded_depth = []
+
+# When deserializing Map<String, Value>, return an error if a key is found that
+# is already in the Map.
+no_duplicate_keys = []
+

src/map.rs

@@ -17,8 +17,12 @@ use core::mem;
 use core::ops;
 use serde::de;
 
+#[cfg(feature = "no_duplicate_keys")]
+use std::format;
+
 #[cfg(not(feature = "preserve_order"))]
 use alloc::collections::{btree_map, BTreeMap};
+
 #[cfg(feature = "preserve_order")]
 use indexmap::IndexMap;
 
@@ -472,6 +476,10 @@ impl<'de> de::Deserialize<'de> for Map<String, Value> {
                 let mut values = Map::new();
 
                 while let Some((key, value)) = tri!(visitor.next_entry()) {
+                    #[cfg(feature = "no_duplicate_keys")]
+                    if values.contains_key(&key) {
+                        Err(serde::de::Error::custom(format!("duplicate key '{key}'")))?
+                    }
                     values.insert(key, value);
                 }
 

src/value/de.rs

@@ -10,6 +10,10 @@ use alloc::vec::{self, Vec};
 use core::fmt;
 use core::slice;
 use core::str::FromStr;
+
+#[cfg(feature = "no_duplicate_keys")]
+use std::format;
+
 use serde::de::{
     self, Deserialize, DeserializeSeed, EnumAccess, Expected, IntoDeserializer, MapAccess,
     SeqAccess, Unexpected, VariantAccess, Visitor,
@@ -122,6 +126,10 @@ impl<'de> Deserialize<'de> for Value {
 
                         values.insert(first_key, tri!(visitor.next_value()));
                         while let Some((key, value)) = tri!(visitor.next_entry()) {
+                            #[cfg(feature = "no_duplicate_keys")]
+                            if values.contains_key(&key) {
+                                Err(serde::de::Error::custom(format!("duplicate key '{key}'")))?
+                            }
                             values.insert(key, value);
                         }