Keep the last of duplicate keys in the object hash index

Lookups in objects with 128 members or more now return the last member of
a repeated key, like the linear search of smaller objects and like
materialize() and basic_json::parse(). build_object_index let the first
occurrence win, so the same text gave different results depending on the
size of the object.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -111,14 +111,12 @@ document.
 
     If the source text has an object with a duplicate key, `#!cpp operator[]` (and [`at`](at.md), [`find`](find.md),
     [`contains`](contains.md), [`count`](count.md), [`value`](value.md), and JSON pointer resolution) all resolve to
-    the *first* member with that key, because a lookup can stop as soon as it finds a match. This is different from
-    [`materialize()`](materialize.md) (and [`BasicJsonType::parse()`](../basic_json/parse.md)), which replay every
-    member in order and so keep the *last* value for a repeated key, so `#!cpp v["a"]` and
-    `#!cpp v.materialize()["a"]` can differ. To get the value `parse()` would give, use
-    [`materialize()`](materialize.md) or iterate the members with [`items()`](items.md) and keep the last match.
-    [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
-    duplicates, in document order. See [Duplicate keys](../../features/json_view.md#duplicate-keys) and
-    [`size()`](size.md#notes).
+    the *last* member with that key. This is the member [`materialize()`](materialize.md) (and
+    [`BasicJsonType::parse()`](../basic_json/parse.md)) keeps, so a lookup in the view and in the materialized value
+    agree. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
+    duplicates, in document order. A lookup in an object without a hash index scans all members for this: it cannot
+    stop at the first match. The hash index of a larger object (128 members or more) leads to the last member of a key
+    as well. See the example below and [`size()`](size.md#notes).
 
 !!! info "JSON pointer resolution"
 

docs/mkdocs/docs/home/architecture.md

@@ -215,8 +215,8 @@ packet-beta
 - **Large objects** (128 members or more) get a hash index after parsing
   ([`detail/view/object_index.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/object_index.hpp)):
   an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does
-  not compare every key. The object's `extra` holds the number of its table. Only 65,535 tables fit into `extra`;
-  objects beyond them are searched linearly.
+  not compare every key. Of duplicate keys the table leads to the last, as a linear search does. The object's `extra`
+  holds the number of its table. Only 65,535 tables fit into `extra`; objects beyond them are searched linearly.
 
 For example, `#!json {"a": [1, 2.5]}` becomes five nodes. Each node's elements follow it, and `next` leads from an
 array or object past its subtree: