How to cancel delivery in clearcase

I want to cancel (undo) a delivery in Clearcase for my Dev stream but it gives below error :

"unable to cancel delivery because another operation is trying to complete it"

What can be the possible cause and resolution?

Unless you are using an old ClearCase 7.0 (whic has a fix to avoid that error), that can happen when the deliver was started twice

The exact error message is:

cleartool deliver -cancel
Cancel deliver
FROM: stream "<source-stream>"
TO: stream "<target-stream>"
Using target view: "<target-view>".
Are you sure you want to cancel this deliver operation?  [no] yes
cleartool: Error: Unable to cancel delivery because another operation is trying to complete.
cleartool: Error: Unable to cancel deliver.

("trying to complete", no "it" at the end)

Attempting to start a deliver twice in the Windows GUI results in a stuck deliver operation.

This is applicable when using UCM with ClearQuest (CQ) integration and having the ClearQuest policy, Transition To Complete After Delivery, enabled.

The CQ policy, Transition To Complete After Delivery, tries to transition the activities to complete, but cannot find any.
This causes the deliver -complete to fail.

More generally, check your OS processes to determine if another process keep an handle which would prevent any cancellation.