Add forbid-duplicated-merge-keys option to key-duplicates rule

tests/rules/test_key_duplicates.py

@@ -179,3 +179,44 @@ def test_key_tokens_in_flow_sequences(self):
                    '[\n'
                    '  flow: sequence, with, key: value, mappings\n'
                    ']\n', conf)
+
+    def test_forbid_duplicated_merge_keys(self):
+        conf = 'key-duplicates: {forbid-duplicated-merge-keys: true}'
+        self.check('---\n'
+                   'Multiple Merge Keys are NOT OK:\n'
+                   'anchor_one: &anchor_one\n'
+                   '  one: one\n'
+                   'anchor_two: &anchor_two\n'
+                   '  two: two\n'
+                   'anchor_reference:\n'
+                   '  <<: *anchor_one\n'
+                   '  <<: *anchor_two\n', conf, problem=(9, 3))
+        self.check('---\n'
+                   'Multiple Merge Keys are NOT OK:\n'
+                   'anchor_one: &anchor_one\n'
+                   '  one: one\n'
+                   'anchor_two: &anchor_two\n'
+                   '  two: two\n'
+                   'anchor_reference:\n'
+                   '  a: 1\n'
+                   '  <<: *anchor_one\n'
+                   '  b: 2\n'
+                   '  <<: *anchor_two\n', conf, problem=(11, 3))
+        self.check('---\n'
+                   'Single Merge Key is OK:\n'
+                   'anchor_one: &anchor_one\n'
+                   '  one: one\n'
+                   'anchor_two: &anchor_two\n'
+                   '  two: two\n'
+                   'anchor_reference:\n'
+                   '  <<: [*anchor_one, *anchor_two]\n' , conf)
+        self.check('---\n'
+                   'Duplicate keys without Merge Keys:\n'
+                   '  key: a\n'
+                   '  otherkey: b\n'
+                   '  key: c\n', conf,
+                   problem=(5, 3))
+        self.check('---\n'
+                   'No Merge Keys:\n'
+                   '  key: a\n'
+                   '  otherkey: b\n', conf)

yamllint/rules/key_duplicates.py

@@ -16,6 +16,18 @@
 """
 Use this rule to prevent multiple entries with the same key in mappings.
 
+.. rubric:: Options
+
+* ``forbid-duplicated-merge-keys`` to forbid the usage of multiple merge keys.
+
+.. rubric:: Default values (when enabled)
+
+.. code-block:: yaml
+
+ rules:
+   key-duplicates:
+     forbid-duplicated-merge-keys: false
+
 .. rubric:: Examples
 
 #. With ``key-duplicates: {}``
@@ -51,6 +63,29 @@
         other
         duplication
     : 2
+
+#. With `key-duplicates`: {forbid-duplicated-merge-keys: true}``
+
+   the following code snippet would **PASS**:
+   ::
+
+    anchor_one: &anchor_one
+      one: one
+    anchor_two: &anchor_two
+      two: two
+    anchor_reference:
+      <<: [*anchor_one, *anchor_two]
+
+   the following code snippet would **FAIL**:
+   ::
+
+    anchor_one: &anchor_one
+      one: one
+    anchor_two: &anchor_two
+      two: two
+    anchor_reference:
+      <<: *anchor_one
+      <<: *anchor_two]
 """
 
 import yaml
@@ -60,6 +95,8 @@
 
 ID = 'key-duplicates'
 TYPE = 'token'
+CONF = {'forbid-duplicated-merge-keys': bool}
+DEFAULT = {'forbid-duplicated-merge-keys': False}
 
 MAP, SEQ = range(2)
 
@@ -92,7 +129,8 @@ def check(conf, token, prev, next, nextnext, context):
         if len(context['stack']) > 0 and context['stack'][-1].type == MAP:
             if (next.value in context['stack'][-1].keys and
                     # `<<` is "merge key", see http://yaml.org/type/merge.html
-                    next.value != '<<'):
+                    (next.value != '<<' or
+                        conf['forbid-duplicated-merge-keys'])):
                 yield LintProblem(
                     next.start_mark.line + 1, next.start_mark.column + 1,
                     f'duplication of key "{next.value}" in mapping')