Comparing columns, creating lists... failing miserably

Hi all,

I am having some issues. Generally my problems are widespread enough that I can just trawl and find something close enough that I can modify to meet my needs. I haven't been able to do so with this issue.

I need to find differences between two columns on multiple sheets, and compile corresponding IDs in a list on a third. I've attached an example document so that this doesn't sound quite as insane as I stumble through explaining it.

Basically, I need to find any differences between columns C and D, then pull the most recent value from column A (variable distance, sometimes this is 1 row above, sometimes it is 10 rows above) and place that value in a list on a separate tab. It would be preferable if I could verify that there are no duplicates (e.g. if there are two differences in the same ID, the ID only appears once), as well as making only one list from several tabs.

The goal is, using this macro, I would have a list in column B on tab "plan" that looks like this:
PE-1
PE-3
PE-4
AC-1
AC-3
AC-4

Any help would be greatly appreciated...