Preventing multiple registration of same hardware device

It works fine for me.

Tested on:
adb 1,0.31 (maybe latest version)
CentOS 7 (kernel-3.10.0-123.6.3.el7.x86_64)
Sony Xperia VL (SOL21) (japanese version of Xperia V)

This conditional is redundant isn't it? The key would just be overwritten if it exists already.