While a different password / key would make the same data decrypt to different data, you don't necessarily have to do de-dupe on blocks of the encryption-cipher-size, so these values can easily overlap.

Blocks of data can be treated completely independently of the encryption. The block size is not necessarily the same as the encryption cipher block size (if you're using a block cipher).

Say, a chunk of 4000 different files all have a few disk blocks that contain the same pattern of data. You can store those few blocks once, instead of 4000 times. With encryption, this is still possible, but it's going to be slower, as the data segments will be more randomly distributed. It is still, however, possible and will result in savings.