Ensure ManyToMany produces a unique schema for the left-side relation

Author: solnic

lib/rom/sql/associations/many_to_many.rb

@@ -20,7 +20,7 @@ def call(target: self.target)
               if target != self.target
                 target.schema.merge(join_schema)
               else
-                left.schema.project(*columns)
+                left.schema.uniq.project(*columns)
               end
             else
               target_schema