diff --git a/doc/src/sgml/func/func-aggregate.sgml b/doc/src/sgml/func/func-aggregate.sgml
index 8b5eaeb2e94..71747bb3c18 100644
--- a/doc/src/sgml/func/func-aggregate.sgml
+++ b/doc/src/sgml/func/func-aggregate.sgml
@@ -404,6 +404,9 @@
         are coerced to text; value arguments are converted as per
         <function>to_json</function> or <function>to_jsonb</function>.
         Values can be null, but keys cannot.
+        If a key appears more than once, the <type>json</type> result keeps
+        every pair, while the <type>jsonb</type> result keeps only the last
+        value for that key.
        </para></entry>
        <entry>No</entry>
       </row>
@@ -432,7 +435,10 @@
         are coerced to text; value arguments are converted as per
         <function>to_json</function> or <function>to_jsonb</function>.
         The <parameter>key</parameter> cannot be null. If the
-        <parameter>value</parameter> is null then the entry is skipped,
+        <parameter>value</parameter> is null then the pair is skipped.
+        Duplicate keys are handled as for <function>json_object_agg</function>
+        and <function>jsonb_object_agg</function>, after removing any pairs
+        with null values.
        </para></entry>
        <entry>No</entry>
       </row>
@@ -490,8 +496,9 @@
         are coerced to text; value arguments are converted as per
         <function>to_json</function> or <function>to_jsonb</function>.
         The <parameter>key</parameter> cannot be null. If the
-        <parameter>value</parameter> is null then the entry is skipped.
-        If there is a duplicate key an error is thrown.
+        <parameter>value</parameter> is null then the pair is skipped.
+        If there is a duplicate key (including pairs with null values)
+        an error is thrown.
        </para></entry>
        <entry>No</entry>
       </row>