I'm not clear on what you're looking for, Nintendo1889.
I understand you want to be able to restore a backup without wiping out existing data.
But what are you using now, Google/Android's default 'Contacts' app?
Regardless, do you have your contacts app automatically backing up somewhere?
Are you trying to merge contacts from more than one account?
You can manually export your contacts, and then import them as needed, without overwriting anything. Would that work?
As I see it, an app that backs up and restores contacts, without overwriting existing data, may add duplicate records unless it's able to read an entire contact, compare its data to a similar one in the backup, and determine whether they're the same or different. I personally don't know of such an app, but someone else might.
Please clarify what you're wanting to do, especially noting if there's more than one account involved.
Edit: ninja! While I was taking my sweet time replying,
@ajhk slipped in a helpful response.