The patron merge tool introduced in 18.05 is a great step, but creates some issues. Contact information for the discarded patron is lost even when the kept patron has null values for those fields. Merging patrons with additional patron attributes results in a patron with multiple values for an attribute that is not supposed to be repeatable. The screen that confirms the merge has completed reports counts of transferred data but doesn't allow one to see details of what was transferred from the discarded patron to the kept patron.
A good next step here would be an interface similar to what we see when merging bibs -- the opportunity to see values for both patron records and select which will be carried forward to the final record, at least for all fields in the borrowers table and for additional patron attributes.
Verified: When we've merged borrower records we've found that the resulting record contains multiple patron attributes (even when these are set as not repeatable).