Don’t do the rebase and you’re set. Simply merge your bugfix branch into each branch you need it
(master)$ git checkout -b bugfix
# do bug fix here
(bugfix)$ git commit -a -m 'Fixed bug.'
(bugfix)$ git checkout master
(master)$ git merge bugfix
(bugfix)$ git checkout experimental
(experimental)$ git merge bugfix
When doing the rebase you are creating a commit similar to the already merged commit, but different. Doing the rebase followed by checkout+merge is essentially equivalent to cherry-picking the bug fixing commit.