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Python, (削除) 34 (削除ここまで) 25 bytes

lambda i:sum(zip(i,i),())

Try it online! Another one that returns a string instead of a list of characters:

for i in input():print(i,end=i)

Try it online!

Python, (削除) 34 (削除ここまで) 25 bytes

lambda i:sum(zip(i,i),())

Try it online! Another one that returns a string instead of a list of characters:

for i in input():print(i,end=i)

for i in input(): asking for the input, and doing a 
 for loop for every char in the string.
 print(i,end=i) print the character, and then close 
 the line with the same character.

Try it online!

Python, (削除) 34 (削除ここまで) 25 bytes

lambda i:sum(zip(i,i),())

Try it online!

Python, (削除) 34 (削除ここまで) 25 bytes

lambda i:sum(zip(i,i),())

Try it online! Another one that returns a string instead of a list of characters:

for i in input():print(i,end=i)

Try it online!