The way you have coded the comparation repeats itself. Besides, that is not the correct way you access an element in a list, I'm afraid.
For example, lets say you have the i = 0 (value 2.0 for example) and k = 2 (value 5.2), but later you have i = 2 (5.2) and k = 0 (2.0), which is the same. I suggest you the following, which avoids this repetition.
for (int i = 0; i < list.size()-1; i++)
for (int k = i+1; k < list.size(); k++)
if(list.get(i) == list.get(k))
System.out.println(i + " and "+ k +" are pairs");
Of course, you have to make sure that the list's size is bigger than 1, otherwise it will throw OutOfBoundsException. Hope this helps ^^
EDIT #2 Try and use the following if, it contains a null-safe equals:
if(Objects.equals(list.get(i), list.get(k)))