I have a lvl 17 Ranger with +2 Half Plate, medium armour master, and a +3 dex modifier. by my calculations, the AC should be 20 (no shield but I am a duel wielder with defensive duelist.

The system calculates my AC as 22....the details appears to indicate the base AC of +2 half plate as 19...but I think that it is actually 17.

I was going to embed an image, but I can't figure out how to include one without having to place the image out on the web somehow (might look into that too.).

Can you post the link to your character?

I can certainly try....not sure if this works but here it is.

https://www.dndbeyond.com/profile/AggieBear/characters/224507

Ok thanks I'll report this to the staff.

Just FYI, with Medium Armor Master and the Duel Wielder Feat while wearing +2 Half Plate your AC will be 21 (17 AC - +2 Half Plate, +3 AC - +3 Dex Mod (Medium Armor Master Feat), +1 AC (Duel Wielder Feat))

Thanks for catching this. Half Plate, +2 had a bonus armor class modifier applied twice giving you the extra 2 AC you weren't expecting.  I've fixed the data and it will take a few for the data caches to update

I don't have the duel weilder feat...it is the duel weilding fighting style for Rangers.

Quote from AggieBear >>

I don't have the duel weilder feat...it is the duel weilding fighting style for Rangers.