Prevents the Future result from being set twice. (backport #1599)

[mergify]

Description

Fixes #1598

Is this user-facing behavior change?

Yes, but this fixes the behavior that has been changed by #1469

Did you use Generative AI?

Yes, Copilot Claude Sonnet 4.5

Additional Information

---

This is an automatic backport of pull request #1599 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a348801 has failed:

On branch mergify/bp/humble/pr-1599
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit a348801.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rclpy/rclpy/action/server.py

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally