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Python, 34 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, 34 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, 34 25 bytes

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

Try it online!

Python, 34 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!