Sign that word 2!

Pyth, 10 bytes

sm@+drd1zG

Try it online: Demonstration

Explanation:

             implicit: z = input string
 m       G   map each letter (variable d) of the alphabet to:
   +drd1        d + upper(d)
  @     z       filter z for these two letters
s            sum, join to a string

Python 3, 72 70 bytes

s=input()
print("".join(d*(ord(d)&31==c)for c in range(27)for d in s))

Assumes the input consists of only [a-zA-Z].

(-2 bytes thanks to @xnor)

Haskell, 51

f s=[x|(a,b)<-zip['a'..'z']['A'..],x<-s,x==a||x==b]

The zip creates a list of pairs of characters [('a','A'), ...('z','Z')]. Because of truncation, the second endpoint doesn't need to be specified. For each pair in the list, we take the letters in the input string s that are either of the two characters in the pair.