Ensure the same item only appears once on the list

[smoy]

Prior to the change, the same item can be repeated added to the list.
We use a map to ensure the same item (based on ID property) appear only once.

[smoy]

It turns out the behavior varies quite wildly between HTC One (M7) with Android 5.0.2 and Nexus 7 (2013) with Android 5.1.1.

With the HTC M7, discovery can report the same item (with same guid) multiples time during the 10sec scan. Where as the Nexus 7 only reports same device during the 10sec interval.

:/

[Yenthe666]

Same problem happens with a Nexus 5 by default (Android 5.0 and Android 5.1). It will keep adding items to the list that have already been added.