Every SP can of course only be delegated once.
You are free to delegate the amount which is displayed on steemd.com as your "Vote Weight" (when your Voting Power is at 100%, if not correspondingly less. If you marked something for power down you have to substract that too).
It is always possible to raise an existing delegation by creating a new one with the old amount plus what you want to add.
You can remove parts of a delegation by creating a new one with less than you already delegated, or 0 to remove it all.