If the employee has more than one appointment record, then it may be that they have two different working patterns assigned, each with a holiday FTE of 1.00.
This will cause the system to give them 100% of their entitlement on both appointment records, which will result in a 200% holiday entitlement being given (double what they should be getting).
When you have an employee with two appointments, they should have their own working patterns, unique to them, set up on the system. The sum of the holiday FTE on each working pattern should not exceed 1.00 (100% of entitlement) unless you agree that the employee should get more than their basic allowance.
A further thing to check is that the career history records within each appointment have the same start date as the appointment.
There should not be a career history record with an effective date that is before the appointment start date, as this will be included in the pro-rating values and may lead to inaccurate holiday entitlement.