Repeat every other character in string starting with second character

sed, 12


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brainfuck, 13 or 11 bytes


Assumes EOF->0. :(

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(Assuming input also continues to give zeroes indefinitely after the first EOF, we can do it in 11 bytes:


though this eats memory.)

Jelly, 3 bytes


A full program accepting a string which prints the result

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Utilises the fact that the implementation of the double atom, , is multiplication by two, and that strings are lists of Python characters. In Python when a character is multiplied by two it becomes a Python string of length two (e.g. 'x'*2=='xx'). Lastly the implicit output of a list composed entirely of characters and strings (or lists thereof) is its content all smashed together.

ḤÐe - Main Link: list of characters  e.g. input abcd -> ['a','b','c','d']
 Ðe - apply to even indices (1-indexed):
Ḥ   -   multiply by 2                     -> ['a', 'bb', 'c', 'dd']
    - implicit, smashing print            -> abbcdd