Once MC19 is licensed, it shouldn't ever ask again.
The license is pre-entered into the box in the Licensing Dialog if it is in your system clipboard (to make it easier to copy/paste it in). So, if when presented with the thing, you went and found the code and did "copy" then that explains why it was already there.
If you successfully licensed it the first time, then I can't explain why it asked again. Even when you uninstall MC it should remain licensed when you reinstall it. The only things I can see messing it up are:
1. If your system time was all out of whack at some point (either when you did it the first time, or in-between).
2. If you used a registry cleaner or one of those "uninstall helper" applications. (Don't do that, they're all pretty much terrible.)
3. If you went in and manually mucked about with JRiver registry keys.
4. If you did a system restore in between (rolling changes back to before the license was applied in the first place).
I was assured that "u can't run out of restores as you can post here and ask for a reset" then I was told not to worry about it....
This is true. If you ever run out of restores for a legitimate reason (as in, you're not giving your license away to friends and family), just ask and they'll reset them for you.
So... Keep an eye on it. Hopefully it doesn't come back again. If it does, let us know, so we can figure out why.