This reverts commit 30372e41e8f8e92557ee9fd965d64b8fee8dc357.

The deduplication won't work for duplicate candidates coming from the same Capf.
Sometimes Capfs produce duplicates which only differ in their properties.

Duplicates created by Dabbrev and Dict are deduplicated, since they behave
identically during completion:

   (setq-local completion-at-point-functions
     (list (cape-capf-super #'cape-dabbrev #'cape-dict)))

However completion candidates which cause expansion due to an
:exit-function (cape-abbrev or tempel-complete) are not deduplicated:

   (setq-local completion-at-point-functions
     (list (cape-capf-super #'cape-abbrev #'cape-dict)))