[img width=425 height=510]http://fc05.deviantart.net/fs7…burstinglight-d5bcmoj.jpg[/img]
IC:
OOC: Stan Twin Theory I do not own this it is owned by Pretty in PWN on Tumblr.
NOTE: To be specific for this post, we’re going to have two Stan’s to refer to.
Twin Number One is the Stan we all know and love, Stanford, which is the Alex-Hirsch-Confirmed full name of Grunkle Stan.
Twin Number Two is the Stan we don’t all know and love (yet), Stanley.
Whether or not that is Twin Number Two’s real name, IDK, but for the purposes of this theory we’ll just call him that.
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Alright, so this theory already establishes that, just like Dipper and Mabel, Stanford is a twin. Before I put in my own speculation, I wish to present evidence for this theory, going chronologically through the series. Here it is:
Evidence Number 1: El Diablo
So, because Stanford is a truly classy guy, he has a truly classy car: a 1967 Ford Galaxy 500 XL.
This thing:
http://www.google.com/imgres?c…Bg&zoom=1&ved=0CDkQhBwwAQ
http://www.google.com/imgres?c…Cw&zoom=1&ved=0CFQQhBwwDg
Notice the license plate. It says: “STNLYMBL”. Which I assume means, “Stanley Mobile”. Here’s the funny thing, though: Grunkle Stan’s real full name is Stanford, not Stanley.
When I first saw it in Legend of the Gobblewonker, I believed that it might’ve been a mistake on the show creators’ part. But throughout the series, the license plate has consistently remained the same, even as recently as Summerween, as seen in the picture above.
So, why in the holy mackeral heck would he get a license plate that said Stanley instead of his full name, Stanford? The answer is obvious. As stated by many GF theorizers before me, it’s not his car.
Keep this fact in mind as we continue on.
Evidence Number 2: The Boxing Companion
This is the piece of evidence is what really got me/blew my mind. As stated before, a wonderful Tumblr user named Maritova posted this screenshot and pointed out a familiar character in the background:
http://www.google.com/imgres?c…Cw&zoom=1&ved=0CEUQhBwwBw
Alright. Now, I highlighted a few of the parts that caught my eye. Firstly, we have the red, purple, and green outlines. Looking at the parts of these two boys’ hair, we can deduce that they’re the exact same shape. Secondly, look at the orange circled parts: they both have that weird hair floof thing (it’s hard to see the background character’s due to size, but you can watch the episode to check for yourself.).
Dipper and Mabel have it, too:
http://www.google.com/imgres?c…Cw&zoom=1&ved=0CDkQhBwwAQ
See? Weird hair floofs. It’s a Pines family thing, I guess.
Not only that, but that Stan clone in the background of the boxing scene has the same hair color, the same skin shade, the same boxing outfit and color, and the same helmet shape and color.
So, I’m guessing that because of these reasons, they’re at least related.
Judging by the fact that they look identical and even have the same damn hair floofs (I’m sorry, I love saying that <3), I’m guessing they’re twins. Fraternal, like Mabel and Dipper. Which is adorable, I must say. 
Wait a second… I’m saying that these boxing kids aren’t identical twins?
Yep. And here’s why:
Evidence Number 3: The Appearance of the Older Stans
In the show itself, we’ve been presented to two younger forms of Stan (ignoring the boxing flashback) that seem to be in the same age range.
Number One, from The Time Traveler’s Pig, looks like this:
https://www.google.com/url?sa=…UXow&ust=1391371824255208
Number Two, from Boyz Crazy, looks like this:
http://www.google.com/imgres?c…CQ&zoom=1&ved=0CDsQhBwwAg