BUG, TST: Check uint64 behaviour in algorithms.py
First of a series of PR's to patch and test `uint64` behaviour in `core/algorithms.py`.  In this PR, the following functions are checked:

1. `duplicated()` : robust but now has test to confirm
2. `mode()` : robust but now has test to confirm
3. `unique()` : non-robust but patched and tested