It failed because it was trying to duplicate a value in a key field which can not have duplicates. If you know that there really aren't duplicates (if the key is an autonumber) then the autonumber may be off and need to be reseeded.
To reseed it look in the table and find the highest autonumber value. Then do an append of ONE record and in that record include the autonumber field in your append and put the number that it is supposed to be for the next record in the field like this:
Field: MyField:722
Table:
...
Field To Append: YourAutonumberFieldNameHere
Then, when you've run the append with that single record you should be able to go back to do the rest of them without including the autonumber field in it and it should append normally.