When you have spotted a duplicate pin here are some steps that will increase you chances for merging them.
Before starting, please check if any of them are claimed by the owners. Claimed places are practically impossible to merge. Just look for the link “Claim this business”, if it’s not there, then the place has been claimed. If only one is claimed you can try to merge the unclaimed place into the claimed place.
1. Write down the names of the two pins and how many reviews each of them has.
2. Figure out which one should be removed = merged into the other one. Typically the one with the most reviews and pictures should remain on the map = the primary one. If one of the pins is claimed it should be the primary. If both are claimed, ask the business owner to fix the problem.
3. Check if the category is exactly the same for both. If not, edit the category of the secondary pin to match the primary.
4. Check if the names are exactly the same. If not, edit the name of the secondary pin to match the primary pin.
5. Check if the street address is exactly the same for the two pins. If not, edit the street address for the secondary pin to match the primary.
6. When these edits are implemented go ahead and Suggest an edit to the secondary pin. Choose “Remove this place” and then select Reason for removal = Duplicate of another place. Then your primary Pin (here the number of reviews comes in really handy) should show up on the list. Please select it. If it is not on the list then abort the removal. Double-check names, categories, addresses, and that the pin locations are relatively close to each other.
7. If the primary still does not show up you can try to make the phone numbers and websites match also as suggested by @Amiran. But please also be patient to allow your edits to be synchronized between servers. Waiting 24 hours might be what you need to be successful.
As a shortcut, you could try removing the secondary pin when only the categories match. You might get lucky and get the primary to show up on the list. If not, you should continue making more and more details match exactly. Due to the need for servers to synchronize it is probably overall faster to first match all details. And then have all edits synchronized in the same 24 hour period. Rather than making it a week-long project.
Good luck.
This guide is part of the Local Guides Clean The Map Project
Bonus tip:
When working on Desktop trying to merge two (or more) places, it can be a hassle to jump between each place to check phone numbers, categories, web address, the spelling of names, etc. Mostly because all pins might not be visible on the map at the same time, so you may need to repeatedly do a text search to jump back and forth. This can take a lot of time.
My new tip is to open each place in each of their own tab in your browser. This way it becomes super quick to jump between them and compare all facts.
Alternatively, you can also use two devices when comparing listings.