Duplicate Entry on custom field and preview

[hazuremon]

Users were complaining about custom field entry got duplicate after previewing. Found out the custom field value are prematurely updated to database without user clicking on "Add Custom Field" and "Publish".

To replicate the bug:

1. New Post
2. In custom field, select a name from drop down menu
3. Enter a value for the custom field.
4. Without publishing or adding the new custom field, press on "Preview".
5. The typed custom field will be added to database and reflect in Preview.
6. Close Preview window and press Preview button again will again post the custom field value as a separate instance, which cause the duplication.
7. Custom field on dashboard did not reflect the duplicate entry until the page is refresh.

Same bug has been affecting all our sites on WP 3.3.0 and 3.3.1. Haven't test on version older than that. Also was test under "Admin" level and on all browsers and OS.

[SergeyBiryukov]

#11465