Hi,

There are some modules available in the store for preventing the same, but they seems to be paid. If you are ready you can check it out. Otherwise you can enable the double approval for the leaves and restrict this criteria.

Other option is to do some custom code and override the action of the approval button and compare the logged in user and check the requesting user and raise warning if both are same.

Python code:

class HRLeave(models.Model):
_inherit = 'hr.leave'

@api.multi
def action_approve(self):
    res = super(HRLeave,self).action_approve()
    # do the coding here to check the user and requested employee and raise warning
    return res

Thanks