Make product attribute name unique

The name of the attributes are the keys of the attributes Dict field of product
so they must be uniquely identified at least per sets.
But as an attribute can be reused on different sets, the unique constraint must
be set on the name.

Closes #13574

modules/product_attribute/message.xml

@@ -6,5 +6,8 @@ this repository contains the full copyright notices and license terms. -->
         <record model="ir.message" id="msg_label_value">
             <field name="text">%(label)s: %(value)s</field>
         </record>
+        <record model="ir.message" id="msg_product_attribute_name_unique">
+            <field name="text">The name of the product attribute must be unique.</field>
+        </record>
     </data>
 </tryton>

modules/product_attribute/product.py

@@ -1,7 +1,7 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 from trytond.i18n import gettext
-from trytond.model import DictSchemaMixin, ModelSQL, ModelView, fields
+from trytond.model import DictSchemaMixin, ModelSQL, ModelView, Unique, fields
 from trytond.pool import PoolMeta
 from trytond.pyson import Eval
 
@@ -21,6 +21,15 @@ class Attribute(DictSchemaMixin, ModelSQL, ModelView):
         'attribute', 'attribute_set', 'Sets',
         help="Add sets to the attribute.")
 
+    @classmethod
+    def __setup__(cls):
+        super().__setup__()
+        t = cls.__table__()
+        cls._sql_constraints += [
+            ('name_unique', Unique(t, t.name),
+                'product_attribute.msg_product_attribute_name_unique'),
+            ]
+
 
 class AttributeAttributeSet(ModelSQL):
     __name__ = 'product.attribute-product.attribute-set'