Hello,
That's something I noticed since I upgraded to 19.04 (didn't happen before): on first boot, after I've power my PC on, when I log into my Ubuntu MATE session, some indicators appear twice in the panel; once in the indicator applet as expected and once in the systray area. They're not always the same and it seems to be completely random (sometimes there's one, sometimes two...).
If I log out and back in, they're loaded correctly.
Is it a known issue and is there a fix?
In the past, screwups in the loadingof panel elements was nearly always down to something called "race" conditions which I usually solved by spacing out the loading of element by creating scripts with sleep commands to slow up the loading of stuff.
It wouldn't surprise me is something similar was happening now
There's still something really weird: the duplicates that appear in the systray are indicators, and at least one of them doesn't have a fallback AFAIK, it's not supposed to be able to appear in the systray.