How did I end up with two of the same dragon priest masks?

Solution 1:

Since there is no in-game method to obtain two of the same type of dragon priest mask in a single game, this is definitely a glitch.

As others have mentioned in the comments, this is likely a bug that was introduced when Hearthfire was added. As @pushasha has noted, it appears that you did, indeed, put your Volsung mask on a mannequin. Thus, you unintentionally caused the item duplication glitch, described here:

Armor Duplication Glitch

Duplicating armor on mannequins: PC Only This bug is fixed by version 1.2.4 of the Unofficial Skyrim Patch.

• Place the armor that you want to duplicate on your mannequin.
• Don't exit out of the equipping screen, then take all of your armor back.
• Leave the area (loading screen) and come back. The set of armor you did the action with will now be duplicated on the mannequin.
• If done to an enchanted piece of armor (e.g. personally enchanted Ebony Armor of Destruction), it will be the unenchanted form. However, armor that was already enchanted when you found it (e.g. the Ebony Mail) will retain its enchantment.
• Tempering seems to be removed from the duplicate armor.
• This even duplicates dragon priest masks.
• It works on armor from add-ons

I have found no other method for duplicating armor aside from this, so it seems quite likely that this is what happened to you.

Luckily, there seems to be a possible fix for people who didn't want their items duplicated (such as yourself), described in this thread:

1. Take all duplicated armor off of mannequin. Leave the area and come back (I went in and out of the cellar for this).
2. Add a full set of unique enchanted armor (Thieves Guild, Nightingale, Shrouded Armor, etc.) and shield (such as Targe of the Blooded), leaving the duplicated armor in the mannequin's inventory. Enchanted gear should appear on the mannequin over top of the duplicated gear. Leave the area and come back.
3. The old duplicated gear should be on top again. Take everything out of the mannequin's inventory. Without leaving the mannequin inventory screen, add all the duplicated stuff and then all the enchanted armor IN THAT ORDER. The enchanted gear should be visible on the mannequin before leaving. Leave the area and come back.
4. The enchanted gear should still be on top. Take everything out of the mannequin's inventory. Go in and out of the inventory several times for good measure, then leave the area and go back.
5. Mannequin should now be naked and have nothing in the inventory. Supposedly it's done duplicating armor now.