mailchimp_lists_load_user_mergevars() lost subscription fields

mailchimp_lists_load_user_mergevars() lost some of 'Merge Fields' if they use the same source fields.

For example if on subscription settings page in 'Merge Fields' block you will set 'name' as a source both for 'First Name' and 'Last Name' one of them the will be lost during mailchimp_lists_load_user_mergevars() call and as a result of this the module will send a request to mailchimp server with missed required field.

That is happen because of this line in mailchimp_lists_load_user_mergevars():

$mergevars = array_flip($mergevars);