I am not sure if 2X is even a valid word.
What are the proper usages for each of the three?
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Firstly, they all mean exactly the same.
2X is used informally and sometimes in technical writing as a convenient abbreviation to avoid verbosity.
2 times highlights the number 2 and may be used to standardise writing instead of using the special twice if there are other numbers involved (5 times, 8 times).
Twice is most succinct in ordinary non-technical writing.