The first way is to go to the duplicate listing and make a suggested edit saying the listing is a duplicate of another. This is the simplest way of merging a duplicate listing but does not always get accepted by Google’s team.
The second way is a bit more complex.
Step 1, You will need to claim both your original listing and the listing you wish to merge. Once you have claimed both listings, go to your Google My Business dashboard and on the bottom left click on the support button. A menu will appear. You will need to click on the Contact us button. In the 1st step, say you are looking to merge a duplicate listing with your original listing.
Step 2, select the Duplicate Business option and click Next Step.
Step 3, you will need to email the Google support team. Fill out the form with your basic information. When you get to the section asking you to explain the situation in detail, this is where you want to be as clear as possible with what you want done to your listings. Make it clear as to which listing is the duplicate and which listing is the real one. Press submit and your request will be processed by Google's support team. Due to COVID, it could take quite some time for Google to process the request but eventually, it will be completed.
As you are waiting for Google to process your request, take note of how many Google reviews your listing has. When a merge happens, the reviews on the original and duplicate listings are looked over. If Google sees fake reviews, they will remove them as part of their merging process. If you lost a lot of reviews in the merge and are confident that your reviews were legitimate, wait about 2 weeks before taking further action. It takes up to 2 weeks for ranks and reviews to fully settle. After two weeks, if you are still seeing a lot of missing reviews, you will need to go to your Google My Business dashboard and open a new support ticket.