You can copy masters within the same document or from one document to another to use as the starting point for a new master. You can also copy masters to other documents when you synchronize documents in a book or import master pages from another document.
Drag the page name of a master spread to the New Page button at the bottom of the panel.
Select the page name of a master spread, and choose Duplicate Master Spread [spread name] in the panel menu.
When you copy a master, the page prefix of the copied master becomes the next letter in the alphabet.
Click and drag the master spread to the destination document’s window to copy it.
Select the master you want to move or copy. Choose Layout > Pages > Move Master, and choose the destination document name from the Move To menu. If you want to remove the page or pages from the source document, select Delete Pages After Moving, and then click OK.