Mac OSX 10.5.2, collanos 1.3.0.2; Build id: 20080327-1240
We are having the same problem here, slightly different scenario: a task is entered in a work space. Since there is no way to show who the task is assigned to (unless you force all tasks into a special folder) the default user behavior is to first choose "All Tasks" from the top menu and sort on name then you see what tasks you have and their status....Now two problems:
1) Just as tasks in work spaces do not show who they are assigned to, tasks in All Tasks do not show what folder they are in.

and since he folders are used as containers for discreet projects, we are a bit "blinded." But if you look in the folder itself you won't see who it is assigned to, harumph...

2) A task that is entered just once in the work space appears at least in once case five times!
Other tasks appear twice: once in their "new" state (with initial notes in notes field) and a second time in their "Completed" state (with original notes and new additions.)
Must be a bug. If it is intended as history, then the latest should be on top and earlier "states" should be locked an you can only edit the latest "version" of the task.. but in fact, you can edit any of them.
this makes the All Tasks pretty much a mess and a misleading environment. Of course one will intuit to try to pick the latest of the duplicates, but in some cases the same task, with status "Completed" appears 3 out of the five times as "Completed"
And, to make matters even worse: this task:
"Documentation for YouTube Option" appeared twice in All Tasks but not at all in the Workspace! Now, if I go to look at our "super peer" which is a machine that is supposed to keep everything in sync. I see a work space that is completely out of date! (compared to what I have in my own Collanos for the same work space)
so the supposed SuperPeer is no updated at all, even though it is sitting here "Green for Online" and it sees me as being on line and I also see it as being on line, but it is not being updated with changes I've made for over a week in my work spaces. and there is nothing in the conflict bin either.
My conflict bin is full of entries... but these are all legitimate changes others have made and were expected to make.
So, I delete everything in my conflict bin. Oh Joy!

all the duplicates in the All Tasks are now gone!
ergo: bug: Collanos should now show tasks in the conflict bin, but only show the latest version of an object regardless of which member modified it.
separate but related issue:
IMHO: Tasks, wherever they appear, and in any and all contexts, should show to whom they are assigned. That is, after all the number one "facet" of a task. Having to push them all into a special "Tasks" folder is understandable from a programming point of view (because you need to be able to have control over the whole table GUI for the entire pane) but from an organizational point of view it's counter intuitive. I use workspaces for an over riding work area and folders for discreet projects within the work space. So one wants to see the tasks for a discreet project in context (inside the folder list) and also see to whom it is assigned. This area of Collanos needs a bit more strategic thought and refinement.
But, for now, at least if we can eliminate duplicates in the ALL Tasks, and show what folder they are coming from, that would be a big help.
Answering your questions if you delete a task that is a duplicate (e.g. an old "version" with Status "New" for the same one with status "Completed" the old one goes away and the new one remains.