QWork does not allocate Assignments on particular days/hour, rather it keeps a record of how many hours in total a Worker has been assigned within a given week.
Using this tally, QWork will then automatically prevent a new Assignment being allocated to a Worker if that would mean the Worker’s total hours for the week would be exceeded, as per the restrictions set within their profile.
It is therefore the responsibility of the Worker to manage their Assignments and ensure they do not accept work at a time that overlaps with another previously accepted Assignment.
When you search for a Worker you can view on their maximum hours cap, the hours they are current commissioned and the remaining hours for the week.
When creating assignments best practice is for the assignment to start on a Monday and end on Sunday, even when the assignment is only for a few hours / days during that duration. This will avoid overlapping of assignment and make it clearer for you to view as a Hiring Manager.