How to encrypt dates using format preserving encryption and avoid collisions
So, in the comments, you state that


  I'm using the original date as the key


This is the reason for the duplicate dates. The encryption is entirely deterministic for a fixed key. When you change the key, however, it is not. So it is entirely plausible that encrypting the number 215 with the key 215 could result in the same ciphertext as encrypting the number 10 with the key 10. But, encrypting the numbers 215 and 10 each with the same key, say 111, should be guaranteed to result in different ciphertexts (assuming you are not using the tweak, or using a fixed tweak).

If you had a fixed, random key, then you should get different ciphertexts. Adding in a random, per-message "tweak" can also cause this behavior.

So you can fix this "problem" by just using the same key everywhere and no tweak. This does, however, scare me. You are using deterministic encryption in an application that has a fairly small domain. Could make you vulnerable to frequency attacks, or other attacks, depending on the exact application.