Shrinking list that can be used multiple times

I am trying to enhance an excel tracker for work but am having issues understanding how to get it done. I have made a list that can shrink after placing selections in a cell from a drop down list, but the issue is I need to be able to use that list over again within the same worksheet on the next column (it only works once, because the list disappears after the selections are made). Essentially, I have dates across the top that signify work dates for posting workers, and I have names in columns that are the workers. the list is made up of the posts or selections that could be assigned to the workers daily. We keep the data in the worksheet up to 6 months and also work ahead projecting posting/workers for up to 2 weeks in advance.


Other considerations are: The jobs/selections cannot be utilized twice in the same column (hence the conditional formatting that looks for doubles). I am trying to make an exception for the list to not shrink on only 'BRK' and 'TRN' selections, as these signify actual off days for workers.

The first worksheet labeled 'Legacy Posting', is formatted to what is currently being utilized. It does not use any sort of shrinking list, but instead uses a data list associated with the job selection list. The second worksheet labeled 'Shrinking List Posting' is the worksheet I am currently trying to develop and has the current formatting/formulas that have been created to shrink lists.

One the second worksheet you can see the selections made for the column 16FEB. The list shrinks as you select them. Once you start to make selections in 17 FEB, you have a reduced list from the previous date. I need to be able to select from the same list I originally started with (with all selections still made available).

Any help that to modify the existing formula I am using, or create a new formula that works would be great. I would prefer to not use macros/VB because I am unsure how to implement them properly. Thank you for any help!