A table can appear only once in a database diagram. However, if you want to create a new table that contains some of the same columns as an existing table in your diagram, you can duplicate the existing table as the first step in creating your new table. Then in your new table, you can delete unwanted columns, add new columns, and assign a unique table name.
Use the same technique for duplicating tables across database diagrams.
To duplicate a table
-or-
Click the New Table button on the Database Diagram toolbar.
A blank table is added to your diagram.