There could be instances where a client of yours has more than one listing in My Business.
Google and people in the forums suggest you keep the best (optimized) listing, apparently having the most reviews, and delete the others.
Now, removing the other listings may work 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.
Contacting Google My Business may be difficult as lately, they changed things leaving few contacting options.
Here is a tip. If you send a Twitter DM to @GoogleMyBiz, they may help you solve the issue.
A person from 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.