Detection of same leaves can be improved

[andytoh1]

Currently, leaves are flagged as being the same if they have the same start and end date for the same employee.

One issue with this is that even after rejecting a leave for the given date range, another pending leave for that date range cannot be added, though it might make sense for an employee to resubmit an application for their desired leave dates after being rejected.

It is also slightly unintuitive that a new leave application can have its duration fall completely within the duration of another approved leave.