Feature-request: Add a function that duplicates a sequence

[Mr-Xcoder]

If I want to duplicate a String (i.e. append a copy of itself to it, e.g.: ABCD -> ABCDABCD), I have to write *2"ABCD. I think this should be a one-byte built-in, like y is to *2<num>. That would be helpful, especially in tasks involving String manipulation.

[isaacg1]

What one character operation do you think this is more useful that, which we should remove?