How can I duplicate a layer in QGIS?

There are two feature requests regarding this (#5899 and #1483). This is certainly a doable feature, and could likely be included in version 2.0, if a developer were interested in adding it; or, a Python plugin developer gave it some consideration.

The #5899 issue also includes some Python code for duplicating a layer (submitted by developer Giuseppe Sucameli):

In the meantime, select the vector layer then open the QGis python console and run:

iface = qgis.utils.iface; vl = iface.activeLayer(); iface.addVectorLayer(vl.source(), vl.name() + "_clone", vl.providerType())

The previous code adds to the map the same sublayer.

It's difficult to do it using a one-line python script like the previous one, but if you know the sublayer name you can just replace vl.source() with vl.source().split("|")[0] + "|layername=my_sublayer_name" where my_sublayer_name is the name of your sublayer.

It appears after running the duplication code, you will have to copy/paste the original layer's style.

The recent versions of QGIS now have a "Duplicate Layer" option. Right-click the layer you want to duplicate, and it'll insert a new copy just below.

Side note: It seems you need to hit View --> Refresh for changes made in the one layer to propagate to the next.