Hi,
When setting up a duty it should default to 1 x member. And once a member is allocated to that Duty, it should not leave an option for any other members to allocate themselves, as the 'assign to me' link a member uses to allocate themsleves does not appear once someone is assigned to that duty.
Can you please double check that perhaps you have not inadvertently set more than 1 x allocation for the duty?
If issues remain, let me know your team name, specific event in question and duty so we can review.
Kind regards,
Team App Support