Highlight And Message Box Duplicate Values Over All Worksheets Within Same Column

I've been looking for a solution to highlight all duplicates within a certain column across all worksheets in the workbook as the entry is made, no button to search for them. I have found bits and pieces, but can't seem to stitch them together to create what I am looking for, still very green with vba.

My workbook is a loading schedule at a warehouse, so there is a tab for each day (the date being the tab name, ex. '12.04.13'.) Tabs are continually added and removed to progress the calendar, and minimize file size. The column I am searching for duplicates in, is column L (or 12, however you wish to identify it.) If a duplicate value is entered, I want at the very least to highlight the value just entered, and the value elsewhere on the workbook, and if possible, have a msg box pop up stating the location of the other duplicate (or at least the tab (date) the duplicate is on.) The message box is more so for an in your face error check, with a built in GPS. Would it also be possible to only search from row 2 to 100, and ignore any further rows on each sheet?

The point of this, is to locate duplicates to make sure an appt has not been double booked, and both entries can be located to verify which entry has the correct information (carrier, delivery appt, etc.)