Just switched to Linux Mint 17.3. Running Kmail as email client. Whenever I move an email from the inbox and put it in a local folder, I receive the SAME email again in the inbox! What gives?

Could this be your problem?

https://bugs.kde.org/show_bug.cgi?id=283682

Someone mentions very similar activity to yours near the end of the page.

Frankly, if my email client did that to me, I would be using another client very sharpish.

Are you not a fan of Thunderbird?

Force of habit really. Have always used Kmail without issue until now... It's been many years without incident.

Yup, I can appreciate that.

However all the effort is now going into Kontact 5 and that is such an integral KDE component that you really want to be running it in a Plasma 5 environment. You don't really want to be going down the route of finding a PPA version of Plasma 5 to install on Mint 17.3.

I have no idea whether Plasma 5 is slated for Mint 18 KDE, which should be available around July/August, but it may solve your problems if it is (and I imagine it probably will be, since KDE 4 development has been frozen and distributions are starting to drop support for it).

In the meantime, I'm not sure whether Kmail 4 will be updated by anybody to resolve the bug or not, but I doubt it. So, if you can hang around until then and install Mint 18, you *could* be fine.

Otherwise I'd think about moving to another client.