http://www.techradar.com/news/amazons-alexa-can-now-recognize-individual-voices-and-users

Amazon just made it a little harder for someone else to hijack your Echo – the Alexa voice assistant can now recognize individual voices and personalize its response, so you and the rest of your family can use your smart speakers without any overlap.

You can set up voice recognition on an Amazon Echo, Echo Dot, or Echo Show by reading aloud 10 phrases to create a voice profile (there's a tutorial video here). Alexa will then be able to identify you across any Echo device and 'most' third-party devices with Alexa attached, Amazon says.