Ok, so assuming these are all credit card transactions, then the answer is that if they used a different credit card, they are assumed to be different customers. 

If I "merge" these two customers, then going forward if they use either of their cards, their transactions will be listed under their new single customer account? 

Yes, I believe that would be correct.  By merging the 2 names into one you are telling it that both those cards belong to the same person.  

When I merge customers, I loose their last name. Also the following note is added into the Other field:

// Merged on 2018/01/29. Following fields were not transferred to the current customer:
dedupe.fieldName.emailID: 59548047

Any ideas what is going on?

@square_marc Hmm, that doesn't look right. Can you call our Support team about this so we can troubleshoot and escalate to our engineers if needed? 

When you merge a customer with two last names you should one last name should be saved, and the note will include information about what exactly wasn't transferred, in this case it should say 'last name' [wasn't transferred]. 

Did this issue with the last name getting dropped ever get resolved? I noticed this today. I manually merged a handful of customer profiles that *did* retain the last name...but then the merge process just started dropping the last name and retaining only the first name. Definitely seems like a bug.

@Wine_Master @square_marc Thanks for your patience! This issue has been fixed by our engineering team.

Can you let me know if you're still seeing an error when you merge customers, or if last names aren't appearing after two customers are merged? 

@Helen

I just mergered two customers and now the customer has entirely disappeared from "customers." Ack...not good! 

@Helen ok, an update. The customer was not deleted but his last name was empty so I had trouble finding him. So the last name is still getting deleted. 

@Helen   Sorry, but the last name is still getting dropped when merging customers. I'd paste a screen shot demonstrating the issue is still occuring, but this forum doesn't allow for it.

@square_marc @Wine_Master Thanks for letting me know! I'll check back in with our engineers. Hang in there! 

Thanks @square_marc! The team is still working to fix the issue causing last names to disappear.

A quick update for you: We have fixed the error that was causing the strange message "dedupe.fieldName.emailID: 59548047" to show in the Other field when merging two customers with different last names. From now on, when you merge customers with duplicate information you'll see "Following fields were not transferred to the current customer" instead! 

I'll be back with an update again as soon as I have one. Sorry again for the confusion today!

@square_marc  Square does suggest some duplicate customers, but it's very few (like two out of thousands in my case). Not sure what matching algorithm Square uses to determine a duplicate customer, but it obviously is pretty granular. Agree it would be nice to have an option open that up to match on first and last name and then you as the user can decide whom to merge. Add it to the 'Square wishlist'....