You need to pass a path to the function, not the MapDocument object.
import arcpy
import arcpy.mapping as map
mxd = map.MapDocument(r"C:\MGIS\Finalproject2.mxd")
mxd.saveACopy(r"c:\MGIS\FINALPROJECT3.mxd")
See http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00s30000000n000000 for more information.
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