What do I do if someone else has already said what I wanted to say?

There are two options available to you:

Agree with the point stated by that individual and expand on it by demonstrating the argument’s relevance in other scenarios. You will have widened the scope of the argument by doing so.

Drop the point and come up with other ones.
Do speak up in the first 4-5 minutes of the GD to prevent getting into a scenario where someone else has already spoken what you want to say. If you wait much longer, someone will almost certainly have mentioned your points.