Merge duplicate Google My Business listings or multiple locations into one.
There could be instances where a client of yours has more than one listing in Google My Business.
Google suggests you keep the best (optimized) listing with the most reviews and delete the others.
How to delete a duplicate Google Business Page
Now, removing the other listings may work up to a point. You need to take ownership of all listings, but it takes time for the verification postcard to arrive and it may not solve the issue.
There are cases where the deleted listings popped up after a few months with no reviews attached to them.
Only Google support can merge two Google My Business listings.
Contact Google support:
Use the support option from within the GMB (left menu).
Then you will have the option to request a callback, chat, or send an email.
A person from the GMB community will probably respond and ask for the GMB URLs for all the listings so they can forward the issue to the support team and they will merge the listings and the reviews.
Read more posts on duplicate locations from the GMB community.
And here is the Google support page on removing locations.