Problem: I have a Form that includes a custom field that accepts multiple values and another custom field that only accepts a single value. If the single value doesn't exist in the multiple value field, I would like to add it using cred_save_data.
Solution: You should get_post_meta on the repeating field, to access an array of values it currently stores. Something like this:
$itms = get_post_meta( $post_id, 'wpcf-all-items', false );
Now $itms is an array of instance values:
Array (  => instance 1  => instance 2  => instance 3 )
Then you can use PHP's in_array function to check to see if the new value already exists. If not, add it. If so, skip adding it.
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