You can create a calculation field like
First() = 0
and drag this to filter, set table calculation as table cross and filter by selecting true.
Hope this helps
You can also remove whatever field(s) you have set to "Detail" on your Marks card, as that is what is causing the multiple entries. On text charts usually whatever you are attempting to achieve by having a field set to "detail" can be achieved the same way by changing it to "Tooltip" instead, especially when you're getting the above undesired result.
Thanks very much, will give it a try and let you know if it worked;)