Duplicate Custom Fields are created

[ChrisBAshton]

Duplicate custom fields are created when populating multiple custom fields at once.

Steps to reproduce:

• Create a New Post
• Add a custom field "custom_1" with a value "test1"
• Add a custom field "custom_2" with a value "test2"
• Save Draft

...you will now see the "custom_2" field repeated twice (both with a value of "test2"), so ought to delete one.