Double-time is not double time
Pylons, 7 5 4 bytes

Picked a random language on TIO used it

46vt


Explanation:

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46    # Stack is [4, 6]

v     # Reverse the stack [6, 4]

t     # take top of stack 4


Doubled:

4466   # Stack is [4, 4, 6, 6]

vv     # Reverse the stack twice so it's the same [4, 4, 6, 6]

tt     # take top of stack 6 and again which is 6 again


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