Should validation of unique fields be handled on database level?

Should validation of unique fields be handled on database level?

References

Reference NO 1

http://stackoverflow.com/questions/15189724/should-validation-of-unique-fields-be-handled-on-database-level

Reference NO 2

http://laravelbook.com/laravel-input-validation/

Reference NO 3

http://guides.rubyonrails.org/v2.3.11/activerecord_validations_callbacks.html

Reference NO 4

http://docs.oracle.com/cd/E25178_01/web.1111/b31974/bcvalidation.htm

Reference NO 5

http://allenbrowne.com/ValidationRule.html

Reference NO 6

http://msdn.microsoft.com/en-us/library/1x2k7ys3(v=vs.80).aspx

Reference NO 7

http://www.filemaker.com/11help/html/create_db.8.19.html

Reference NO 8

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