You're right about copying everything, there HAS to be something to compare to to accomplish this through conditional formatting.
So, let's say everything in column A is duplicated in column AA. As you make edits in column A the values would no longer match and you could highlight the cells via that fact.
So, highlight column A and use this CF setting:
Condition1: Formula Is: =A1<>AA1
Format... Pattern: Yellow
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