Issue with conditional formatting in excel

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You wrote: I applied the rule before adding anything into the column and so far it works as it should. will use it and open and close a few times throughout today day and see if it sticks this time.

I'm going to suggest that you reverse your thinking a bit here. You have been describing this from the beginning as a failure of Excel to consistently apply a conditional formatting rule for duplicate cells. Your most recent post (excerpted above) still talks of whether "it sticks this time."

The reality is that likelihood of Excel failing in this case is far, FAR lower, than is one of the other causes that  @mtarler and I have alluded to.

•  @mtarler  suggested the possibility of your inadvertently breaking up the range to which the rule applied, doing so by inserting a row, or some other procedure.
• all of my questions (OK, most of them) were designed to uncover the possibility that what you thought was a duplicate or exact match wasn't in fact. Computers are very precise; we humans are less precise. 
• For example, if you had text entries you might see This as the same as This  but the computer would not see those two entries as duplicates. The latter has a trailing space, easily overlooked by our eyes and minds, which are focusing on the word itself. So if the entries into the cells are being done by typing, i.e., by humans who easily can insert a space without even realizing it, that could be what's happening.
• Similarly, depending on how the cells are formatted for numeric displays, or dates, it's entirely possible for entries to look like duplicates to the human eye/mind, but in fact not be duplicate in the computer's way of seeing.

So when this happens, don't think "why has Excel failed to see duplicates?" Instead, start your investigation with the mindset  "Excel is telling me that these cells are not duplicates, even though they look to me as if they are; so let me see if I can figure out how they're not duplicates after all."