It is advisable to make change in Rule group - rule group the option "Check Includes Unposted Requests" is checked - can we unchecked it ?? NO don't do this option
In your error message, you ll always get the dates mentioned, so you need to look in rptarqdbview for the conflicting absences, there ll be always available.
IT2001 only has posted ones, so better check in above report which might be in sent or approved status
Yeah the deletion is always needs to be done, or reject the sent leave as it collides with existing one, manager will be presented with the error or warning message and they can take appropriate step
You can never have conflicting absence on same time!