Delete field's machine-name in Drupal 7

I'm trying to create a new field, but Drupal came up with the well-known error:

The machine-readable name is already in use. It must be unique

I'm sure that the field doesn't exist anymore (I checked the fields' list and it doesn't show up).

I want to use that specific machine name because I already have developed some functions that are using that name.

So I guess the name is still saved in the database, there is a way to delete it without screw everything up?

Solution

I have used this answer to manually delete the field from the database and it worked!

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