Patch 7.00 has added a sidebar icon to the UI, when the player is playing Arc Warden (screenshot below).

The second icon in the sidebar is identical to Arc Warden’s hero icon, even though the Tempest Double is visually distinguishable.

As a comparison, Lone Druid’s Spirit bear has received a unique sidebar icon when patch 7.00 & its redesign had been released (screenshot & comparison below).

Additionally a recent update added a unique minimap icon for the Tempest Double, but it did not include a hero icon for the Tempest Double (which could be used for the 2nd sidebar icon).

Arc Warden’s 2nd sidebar icon should have a unique Tempest Double hero icon (similar to AW’s, just darker/red eyed), instead of simply using Arc Warden’s regular hero icon (just like the Tempest Double has a unique minimap icon).

AW’s sidebar icons:
LD’s old sidebar icons:
LD’s new sidebar icons: