Your task is marked as a General Task. This means that any user is a possible agent of the task. When the work item is created, the workflow engine attempts to resolve the actual agents, but if it cannot do so, it will send all the work items to all of the possible agents. In your case that is probably everyone.
Lets say there are 1000 users in your company. Every one of your users receives an e-mail in their Outlook notifying them that they have a work item. One of those users logs in and executes the work item and completes it. When the other 999 people log in to their inbox, the work item has already been completed and is no longer there.
Your approach to correct this should be to correct the agent routing rules so that only people that should receive the work item receive it.
Desktop Workplace also addresses this issue. It notifies users when they have a work item in the SAP Inbox (Business Workplace), but one of its nice features is that it will also remove all the work items out of other inboxes when the work item is complete. In your case, this means that the 999 people will have the e-mail removed automatically when the work item is compelted by that first person.
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