Check multiple lines for same string entry

[PlanningDev]

Currently I have a dimension with lines 1-10. The idea is that a user enters a string into the string entry cell and then this drives lookups of data etc. The issue I have is that I don't want a user to be able to enter the same string twice, IE user enters A for line 1 and then A on line 2. Its not as wide open due to the fact that I validate the string entry to make sure it is the same key as an item from a dimension.

I would have used picklists but long picklists don't render in TM1 Web in 9.5.1

Im thinking that I need a hidden cube that would contain both dimesions, string validated dimension and rows 1-10 and then use a rule to supply a 1 for matching items. Then use a rule in the original cube checking that the consolidated number from my check cube is <1.

Does this make sense? Hard to explain in words.