Enthuware

int[] scores = { 1, 2, 3, 4, 5, 6};
System.arraycopy(scores, 2, scores, 3, 2);
for(int i : scores) System.out.print(i);

I don't understand how the correct answer can be 123346. Signature for arraycopy is:
public static void arraycopy(Object src,int srcPos,Object dest,int destPos,int length)
System.arraycopy(scores, 2, scores, 3, 2) is in English: copy 1 (2-1) item from the 3rd position (index 2) of the 'scores' array to the 4th (index 3) position of the same array.
So from 1,2,3,4,5,6 I copy item with value 3 to position 4 and I get 1,2,3,3,4,5,6. Array is immutable and I assume that 6 'falls off'. This gives me 1,2,3,3,4,5 and not 1,2,3,3,4,6

I guess it copies the nr of chars from the pos indicated by 2nd arguments to position indicated by 4th element and overwrites whatever is there...

Your code prints 123346 because from the source elements are 3 and 4. You are putting them in position 4, 5, which makes the final array 123 34 6.