14 reckless fireweavers.
Not sure how we went from talking about dupe mythics after 1st bought to how many of a common. But I will bite. I have 35 bloodsuckers and I do not spend a lot of money on this game.
Just curious to see if there is a most common duplicate
31 Apothecary Geists
Paging @shteev, to hear about his Hitchclaws.
I stopped converting in February so my most frequent since then is 17 wild wanderers. In AKH I’ve managed to get 13 consuming fevers already.
Joraga Invocation is by far my most common dupe. I don’t even spend that much money on the game and I’ve pulled that junk card at least 20 times.
I used to convert dupes for runes to level PW prior to Kaladesh.
Night Market Lookout & Peema Outrider, both x31
Rare or better: Solemn Recruit x10
Mythic: Architect of the Untamed x5
My mythic collection rate varies by set, but is about 50% overall. 88% Rares. Still zero masterpieces.
14 Cathartic Reunion
37 Peema outriders
50 Weldfast Monitor
16 Aether Thoerists.
Most frequent
Common:
33 Ondu Greathorn, Shatterskull Recruit
Uncommon:
27 Village messenger
Rare:
9 Deepfathom Skulker, silverfur partisan, always watching
Mythic:
6 Soul Swallower, Kozilek’s Return, Ishkanah Grafwidow
0 Masterpieces
but I have many many unopened packs, so I may have more from the more recent sets I haven’t opened yet.
You have bested me… I only have 48 Hitchclaw Recluse 
13 Revolutionary Rebuffs. I don’t spend any money on packs. I’ve converted before but couldn’t tell you when. Next highest is 10 Hightide Hermits.
58 Salivating Gremlins
Granted, I really didn’t stop converting until just before amonkhet.
But still I have
27 wander in death and
16 viziers