For your info, there is no limit on how many time staff appraisal to be done.
Most of the company only do 1 time because it is troublesome (include me). But in actual process, staff appraisal need to do at least once in a year for every staff.
So there is no reason for the staff to unhappy with it, in other wise, he/she should be glad because staff appraisal normally for confirmation, bonus, go for better position or increment.
Try to explain to the staff about the purpose of staff appraisal.