better way to duplicate a layer using ogr in python?

Use Fiona of Sean Gillies , a very simple wrapper of the OGR library (The Fiona User Manual)

All the elements of a shapefile (schema, records) are processed using Python dictionaries:

schema of one of my shapefiles as example:

{'geometry': 'LineString', 'properties': {u'faille': 'str:20', u'type': 'str:20', u'id': 'int'}}

one record in the shapefile:

{'geometry': {'type': 'LineString', 'coordinates': [(269884.20917418826, 151805.1917153612), (270409.89083992655, 153146.21637285672), (272298.05355768028, 154047.38494269375), (272941.74539327814, 155484.96337552898), (272169.31519056071, 156117.92701386689)]}, 'id': '1', 'properties': {'faille': u'de Salinas', 'type': u'normale'}}

so to duplicate a shapefile:

from shapely.geometry import mapping, shape
import fiona
# Read the original Shapefile
with fiona.collection('original.shp', 'r') as input:
    # The output has the same schema
    schema = input.schema.copy()
    # write a new shapefile
    with fiona.collection(''file1.shp', 'w', 'ESRI Shapefile', schema) as output:
        for elem in input:
             output.write({'properties': elem['properties'],'geometry': mapping(shape(elem['geometry']))})

If you want to split a large shapefile into many smaller ones, everything takes place in the for loop but all the schemas of the original shapefile are preserved in the dictionary with schema = input.schema.copy() and {'properties': elem['properties']

see How do I find vector line bearing in QGIS or GRASS? for an example of

1. spliting a shapefile
2. preserve the attributes of the original shapefile in the splitted shapefile
3. and add a new field in the splitted shapefile

For Mac OS X or Linux users, it is easy to install. For Windows users, use the version of Christoph Gohlke Unofficial Windows Binaries for Python Extension Packages