Dual Quality items are items with two qualities. They are always in strange and then another quality. The qualities I will be focusing on is, Collector's, Genuine and Vintage. There are Haunted and unusual dual quality items as well, but with haunted I wasn't able to find strangifiers for them all that much. With unusuals I won't count because there are so many and you can unbox them as a dual quality. Normal, Unique and Community won't be included as well. Normal and Unique are glitches while all community items are either stock weapons or hats that don't have a strangifier. Thought Strange Self-Made is interesting that'll be an Bonus Section.

Strange Collector's are the most common dual quality items. Just get a collector's item or make one with a chemistry set. To make it strange, you'll need a strangifier for that item.

These are the all the available item in Strange Collector's

Weapons
-Pretty Boy's Pocket Pistol
-The Cow Mangler 5000
-The Air Strike
-The Righteous Bison
-The Phlogistinator
-The Manmelter
-The Third Degree
-The Widowmaker
-The Pomson 6000
-The Vaccinator
-The Classic
-The Cleaner's Carbine

Cosmetics
-The Private Eye
-The Boston Boom-Bringer
-Toss-Proof Towel
-Dark Age Defender
-The Bird-Man of Aberdeen
-Teddy Roosebelt‎
-Sight for Sore Eyes
-Fancy Dress Uniform

Strange Genuine items are a bit more rare compared to Collector's but the process in making one is the same, you just need a genuine item and a strangifier for said item.

These are the available items in Strange Genuine

Weapons
-Righteous Bison
-The Widowmaker

Cosmetics
-Foppish Physician
-The Anger
-The Merc's Pride Scarf
-Camera Beard
-Archimedes

Strange Vintage items are the rarest out of the three. with it only have two items, both being cosmetics.

Cosmetics
-Camera Beard
-Stockbroker's Scarf

Strange Self-Made items can exist, just like the other three qualities all you need is a strangifier of the item. But to get a Self-Made item, you have to submit an item to the steam workshop and manage to get the item approved by Valve/The Development Team for the next update and then you will be gifted the item in the Self-Made Quality. though it is unknown to me if you have to ask for them to make a strangifier for the item or if they do it at random. I hope this guide helped you if you are interested in getting Dual Quality items!