The friend who keeps pushing after your first no is telling you something about the friendship.

How someone treats a no tells you more than how they treat a yes. Pushing past the first one is a small statement that your answer was an opening bid rather than an answer, and that they expect persistence to work because it has before. One nudge is human. A pattern of nudging is information about how much your word counts with them.

Notice the next time it happens and hold the same answer without adding reasons. If they keep going, name it plainly: I already answered that. You are allowed to be less available to people who need telling twice.