This depends on the extension. Details below:

There can be duplicate devices between the base audience and extended audience. the Household Extension contains ALL the mobile devices associated with a household. When creating an audience and publishing the regular and HH extension during the publish process, there will be overlap (duplicates) between the devices identified in the base audience and any that are also associated within the household.

There should not be as many duplicate devices between the base audience and the extended audience as the system is pulling in real-world friends, family and co-workers from the base audience. There could be some duplicates if two devices that the system identifies within the same social circle are observed at the Geoframed location in the base audience, but there will be less overlap than the Household extension.

No, this extension should return both behavioral and demographic lookalikes to the base audience, but should not include duplicate devices.