Sony started the Android 5.1.1 Lollipop release for the Xperia M2 and Xperia M2 Aqua handsets yesterday, however Xperia M2 Dual (D2302) owners were left out of the party. Well good news is that Sony has now released the update across most regions.
Android 5.1.1 Lollipop on the Xperia M2 Dual has the same build number (18.6.A.0.175) as the other Xperia M2 series. Check PC Companion to see if the update has hit your region, or you can download the FTF firmware file below.
Thanks Craig, galaxyfreak and Henry!