Quickbooks and Quickbooks Online don't allow 2 customers to have the exact same name. This makes sense, or else it would be really confusing to review your customer list.
Because your customer doesn't exist (yet) in Quickbooks, you can't match either of the new estimates to a new customer. What's happening is that the customer is being auto-created by LMN after the first estimate, then it's failing on the 2nd estimate because the customer already exists with that name... and it was impossible to 'match' that estimate to an existing customer before your export because the customer didn't exist yet.
The easy solution is to just re-export your estimates immediately after getting that Partial Success error message. The estimate(s) that didn't get imported will still remain in the queue and now that the customer exists in Quickbooks, LMN will auto-detect a match and everything will go smoothly. It just takes a few extra seconds to run the routine a second time.