If I make a branch 35.1-extra from the 35.1 branch (release branch obviously), do some work, and then after some time merge stuff from 35.1-extra back into 35.1, is it OK to merge more than once?
I remember this was problematic in old versions of Subversion so whenever I reintegrate, I either created a new branch if I had to do some extra work, or re-created the "extra" branch, and continued working there.
You had to keep a reintegrated branch alive with Subversion 1.7 and older clients. Thanks to the new "automatic reintegration merge" feature in Subversion 1.8 release, so called "keep-alive dance" is no longer necessary:
svn merge automatically detects whether reintegration merge has to be done,--reintegrate option of svn merge is deprecated in SVN 1.8+.