The statement "This addon is already installed" does not indicate on my Windows box that I've the latest update as I've just backed my A2Plus Wb to a version from 23/12/2018 and it states "This addon is already installed" without making any indication that it's actually 3 months out of date and that's using the 0.18 RC build:
OS: Windows 7
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16093 (Git)
Build type: Release
Branch: releases/FreeCAD-0-18
Hash: 690774c0effe4fd7b8d2b5e2fb2b8c8d145e21ce
Python version: 3.6.6
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedKingdom (en_GB)