I am wondering if it is possible to have a flag or error message, when the
same cell contents, is entered into two different cells on a sheet.

This is for a master schedule created in Excel.

Example.

The address "52 chestnut" is entered into a cell and remains there.

Later in the sheet, if "52 chestnut" is entered, some sort of error or flag
or the "#######" message can come up.

I know its a strech, but is it possible?

Regards,

Andy