Sometimes it’s the little things going wrong that collectively over time start to become annoying. Contacts spread over various accounts in one such a thing (for me). If that’s you as well, here’s how I fixed it.

A couple of assumptions here before you start.

You’re living in Apple land; we’re talking Mac and iOS here and use iCloud to sync your contacts between your iPhone and Mac.
You have Google Account(s) where many of your contacts also live.
You’ve already decided where you want to have that one place that stores all your contacts.
You want to remove duplicates in the process.

If that’s you, let’s go!

Merge all contacts from various (Google) accounts into your iCloud contacts

Log into Google Contacts
Navigate to Google Contacts and log in.
Go to Export
In the left-hand menu, click on the Export link. A modal looking like this will pop up.
Select Contacts
You could select a specific subsection of your contacts, but since the goal here is to move everything over to iCloud Contacts, go ahead and select “Contacts”.
Export as vCard
This is the most important step if you’re moving to iCloud Contacts. Select vCard (for iOS Contacts) as that will produce the perfect format for your Contacts app on your Mac. Go ahead and save the export file on your Mac.
Repeat this process for other (Google) Accounts
If you are as disorganized as I was, you’ll have multiple Google Accounts where your contacts live. You’ll need to repeat these steps for all accounts to produce multiple exports.
Stop synchronizing contacts on your Mac.
Before you go perform the actual import, it’s a good idea to go into your Internet Account settings on your Mac and turn off contact syncing. You’ll have to do this for all your (Google) Accounts there. You can do this by clicking on the account and uncheck the checkbox for Contacts. As seen below.
Stop synchronizing contacts on your iPhone
You’ll need to do the same thing on your iPhone. Go to the Settings app, scroll down until you see the Contacts settings. Click on that, and then Accounts. There you’ll want to select the accounts you want to stop synching. Again, by deselecting the checkbox there.
Import export files into Contacts
Now for the fun part. Open your Contacts app on your Mac. Once the app is active, go the menu option File > Import. This will prompt a Finder window allowing you to navigate to the folder where you downloaded the exported file(s).
Remove duplicates
Once you’re done importing all your contacts into your Contacts app, you’ll most likely need to do some cleaning up. The Contacts app has a built in solution to find duplicates and merge them into one. You’ll find this in the menu of the Contacts app: Card > Look for Duplicates

Be sur to follow the instructions after you’ve clicked on that menu option, but it should result into a cleaned up version of your contacts.
Make sure syncing is turned on
Of course, just as you’ve disabled synchronizing of contacts for your now old accounts, you’ll need to double-check your contacts are synching with iCloud on both your Mac and iPhone. You can find those settings in the same location per the previous steps.
Delete contacts in old accounts
If you’re like me, you don’t like stuff lingering. So, to remedy that in this context, you’ll want to go into each of those old contact accounts and delete all contacts there.