Currently, DV allows constituents to create multiple records with identical critical field data such as email, phone number, and mailing address. For example, I just now had to merge two records for the same constituent who was in our original data import but also later made an online donation. The system created the new constituent record even though her last name, email, and most of the postal address were identical. (She used a nickname rather than her legal first name when making the donation, and entered a 5-digit ZIP instead of the 9-digit ZIP-plus-4.)
Allowing such records to be created without flagging them as possible duplications is a serious weakness in the system. I understand why DV would not want to halt the donation processes and start asking a potential donor if they are the same person we already have on record, although some systems will do this, usually indicating that the email address is already in use. But if DV is going to allow these overlapping records to be created, they should be flagged and a report should be pushed out to the database administrator to check the records for a possible merge, or correction — e.g. if a new donor has a postal address or phone number which used to belong to a different constituent who has moved.
Here's an example of how a failure to prevent overlaps can create problems. I was trying to clean up a record for a couple which duplicated pre-existing records for each of the two constituents before merging the records. But I wasn't allowed to save any changes because of the duplicate emails. The system shouldn't be deciding for me which of the two records I'm allowed to edit.