Find out from two different columns, which data is old, solved or new

Hi,

I have met a problem recently. Here it is:

I have two columns, which have different length and in which there can be three types of data:

1. The data also in the first and the second column, so it is an old data.
2. Second is, that the data is in the first column, but not in the second, so it is old, but it was solved.
3. The third type is, when the data is in the second column, but not iin the first, so it is new data.

My task is to regularly update the list (so I have to merge the new and old columns), to see in which conditions the data is, but not to delete any of them (only the recurring, so if it is old, then it must not be listed two times). Your help would be greatly appreciated!

Thanks,
Daniel