When responding to an email, most email clients prepend the subject with Re: . K-9 Mail does the same.
But I am wondering if there is a way to avoid prepending that text, and just reuse the same subject as the original email? I am not seeing a setting to control that so far. Thanks for any pointers!
I don’t think this is generally a configurable setting. I just looked at two other mobile device email clients and a couple of desktop ones too. Of these only one (hyper-configurable) desktop client gave the option to adjust that. You can obviously delete the “Re:” (or Fwd:) from the subject line when you are composing your reply/forward message.
The lack of the “Re:” context is likely to confuse recipients and possibly sorting and threading in certain cases.
What problem are you trying to address?
I am a member of a Topicbox community. Responding to a topic creates a new Topicbox thread because of the change in subject. If the subject remains the same, my email response is added as a comment to the existing Topicbox thread.
Based on the discussion so far, it sounds like email clients adding Re: is a standard and it’s Topicbox that must consider updating their logic to include threads with Re: . 
Yes, the “Re:” or “RE:” (for english language client settings) is more or less the standard marker on a reply. Many email clients ignore it when sorting by subject so that the original and replies all sort together. They also generally don’t add another “Re:” if one is already there. The Topicbox developers might want to look at common practices and adjust things accordingly if they want their application to work the way they seem to desire.
Hmm… It is a bit more complicated than that. Some email clients use localized “Re:”. I think some research is needed by anyone implementing “Re:”-stripping.
As I noted, the “english language client settings” are "Re:’ or “RE:”. Non-english localized will be different. See the discussion and some examples at the bottom of:
https://en.wikipedia.org/wiki/List_of_email_subject_abbreviations