Rdoc-data vs rdoc_osx_dictionary

I’m still having trouble with rdoc. When I try to run rdoc-data, I get a
message like this:

can’t activate rdoc (~> 2.5, runtime) for [“rdoc-data-2.5.3”], already
activated rdoc-2.4.3 for [“rdoc_osx_dictionary-1.3.1”] (Gem::LoadError)
from /Library/Ruby/Site/1.8/rubygems.rb:246:in activate' from /Library/Ruby/Site/1.8/rubygems.rb:245:ineach’
from /Library/Ruby/Site/1.8/rubygems.rb:245:in activate' from /Library/Ruby/Site/1.8/rubygems.rb:1056:ingem’
from /usr/bin/rdoc-data:18

I don’t know what all this “activate” is about, or why we are using an
outdated version of rdoc. I have updated rdoc, but my version of the
rdoc binary is still 2.4.3.

Does this ring a bell with anyone? How do I get out of this? Thx - m.