Blogger is a confusing platform in that the search engines see duplicated content across multiple subdomains.
Even with one url it may still list your post as, home, archive, and keyword as three separate entities so adding a new url will double that. Sometimes it's better just to keep an eye on both url' s in case an issue should ever arise,
Like I say I would add the new url, and by watching the metrics you will see the blogspot url die and your new one come to life as it starts to take the authority and people recognise it.