You can only get one helmet per week from the bounties, so if you've already completed this week's helmet bounty then you are done for this week. Unfortunately MoT ends tomorrow at reset so you will be unable to complete that Triumph.
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Yeah I know that you can only get one a week but i already had the second helmet and I just did my third week for the same type of helmet and no helmet dropped.
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Unfortunately there is nothing that can be done this week. https://www.bungie.net/en/Help/Article/12211