How can I eliminate duplicates before I do a backup?

Episode 1058 (1:25:15)

Val from Valencia, CA

Val wants to offload his data onto one large drive and "de-duplicate" his files. Leo says there are a number of programs that can do it, but it's something that makes him nervous because they have the potential to eliminate what it thinks is a duplicate when it isn't. A better way is to scan the contents with a redunancy check. Leo says that SyncToy is a good one.

Leo says he can keep the old drive anyway. Drives are cheap, so he could just use that as a backup.