Draft with translations causes critical build error

If there are files with same slug in draft status.

test_en.md

```
Title: Test in English
Date: 2013-10-4
Slug: test
Lang: en
Status: draft
```


test_ja.md

```
Title: 日本語テスト
Date: 2013-10-4
Slug: test
Lang: ja
Status: draft
```


This causes build error

```
CRITICAL: File /path/to/blog/output/drafts/test.html is to be overwritten
```

pelican version is 3.3.0