Fix bug: unexpected duplicate postmeta data

Duplicate Post plugin copies metadata, but afterwards, CFS creates new cfs field data. so result is...

original post:
![original_post_meta](https://cloud.githubusercontent.com/assets/514294/3560718/cc9f6514-0985-11e4-8679-bf1092f54ba6.png)

post copied with Duplicate Post plugin:
![cfs2](https://cloud.githubusercontent.com/assets/514294/3560720/d74be618-0985-11e4-9260-048468bd9e99.png)

![cfs3](https://cloud.githubusercontent.com/assets/514294/3560722/fb00f314-0985-11e4-80d6-17f2799cd133.png)