There is no limit to the number of times you can complete a single task. If you are not allowed to continue with the same task a second time, then it may be for the following reason:
For various reasons, tasks set an upper limit on the amount of responses a single worker can submit.
Very few participants ever reach this limit.
If you do, then please be aware that it does not reflect poorly on the quality of your contribution, and you are still free to work on any of the other tasks on the list.