Function outputs in Excel spreadsheets update automatically -- and silently -- whenever you change a cell mentioned in the function's formula, so every referenced cell needs current information. When you have two columns that share duplicate data, linking them, rather than merely copying and pasting their contents, ensures that the info in both locations stays up to date. Excel allows you to link columns on the same spreadsheet, between sheets in the same workbook or between two separate workbook files.
Select the lettered header of the source column -- the column that contains your data -- to highlight the entire column. If you already filled out the same data in two columns, it doesn't matter which you pick as the source.
Press "Control-C" or right-click the header and pick "Copy."
Right-click the header for the target column where you want to create the link. It's okay if the target column already has the same data in it; the link overwrites any existing content.
Click "Paste Link" in the Paste Options section of the context menu. Its icon looks like a chain link. The entire column will fill with data linked from the source column, with the side-effect of displaying a "0" in every cell after the end of your data.
Select the first extraneous "0" and press "Control-Shift-Down" to highlight the remainder of the column. Press "Delete" to remove the unnecessary links and get rid of the zeros. If you plan to use more rows of data in the future, you can start a few cells lower and leave some of the zeros in place. Do not leave the entire remainder of the column filled with zeros, however, as this dramatically increases the file size and slows down operations.
After creating a link, always update data in the source column. Typing directly into target column erases the link.
To link additional rows you previously removed the links from, click the lowest linked cell and drag the fill handle downward to cover each cell you want to link.
When linking columns in two separate workbooks, both workbooks must remain on the same computer (or accessible over the network) to keep the linked data updated. If you rename or move one of the workbooks, you will have to recreate the link.
Information in this article applies to Microsoft Excel 2013 and 2010. Procedures may vary with other versions or products.