Python 3, 44 43 41 37 25 Bytes

Edit: I have refactored it a ton and came to this answer:

print(*[x*2 for x in input()],sep="")

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then from there I got to

lambda i:[x*2 for x in i]

The TIO doesn't work properly due to it being a function but here it is anyways: The TIO now works thanks to des54321 for their advice:

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Here's my other implementation of this challenge with greatly appreciated help from the comments:

print("".join(map(lambda x:x*2,input())))

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Description

It works by separating the script using pythons map function to grab every character in the inputted string then multiplying them by 2 and joining them back together after it is converted to a map using an empty string then joining every element to it.

edit: thanks to pxeger for suggesting to remove unnecessary spaces, to Wheat Wizard ♦ for suggesting to remove the quotes from the input and to des54321's advice of using f=\ in the header to fix TIO with lambda functions.