cua-paste-pop-rotate-temporarily is a variable defined in `cua-base.el'.
Its value is

If non-nil, M-x cua-paste-pop only rotates the kill-ring temporarily.
This means that both C-y and the first M-y in a sequence always insert
the most recently killed text. Each immediately following M-x cua-paste-pop replaces
the previous text with the next older element on the `kill-ring'.
With prefix arg, C-u M-y inserts the same text as the most
recent M-y (or C-y) command.

You can customize this variable.