Fix double snapshot creation error

lib/nanoc/base/compilation/item_rep_recorder_proxy.rb

@@ -70,7 +70,7 @@ def snapshot(snapshot_name, params = {})
 
       # Count
       existing = Set.new
-      names = @rule_memory.select { |r| r[0] == :snapshot }.map { |r| r[2] }
+      names = @rule_memory.select { |r| r[0] == :snapshot }.map { |r| r[1] }
       names.each do |n|
         if existing.include?(n)
           raise Nanoc::Errors::CannotCreateMultipleSnapshotsWithSameName.new(@item_rep, snapshot_name)

test/base/test_item_rep_recorder_proxy.rb

@@ -0,0 +1,21 @@
+# encoding: utf-8
+
+class Nanoc::ItemRepRecorderProxyTest < Nanoc::TestCase
+  def test_double_names
+    item_rep = mock
+    proxy = Nanoc::ItemRepRecorderProxy.new(item_rep)
+
+    proxy.snapshot(:foo, stuff: :giraffe)
+    assert_raises(Nanoc::Errors::CannotCreateMultipleSnapshotsWithSameName) do
+      proxy.snapshot(:foo, stuff: :donkey)
+    end
+  end
+
+  def test_double_params
+    item_rep = mock
+    proxy = Nanoc::ItemRepRecorderProxy.new(item_rep)
+
+    proxy.snapshot(:foo)
+    proxy.snapshot(:bar)
+  end
+end