Correct translation string

There is an extra "額" in "可以額外啟用下列額字元" in the configuration
page, the second "額" should be eliminated. It is now.

This commit fixes issue #62.
Merge pull request #62 from yookoala/translation_correction

Correct translation string
engine: Deduplicate characters

This avoids having twice the same candidate in the list presented to the
user.

Fixes #63