You may have more than one instance of each plugin running at Start up. Can you check your startup list?
It looks like nm-applet (network manager). I am not sure what the other one is... Brightness? Redshift?
You might also check if you have notification area and Status-Notifier added to the panel.
Right click panel > Preferences > Items tab.
Is indikator bovitmeny (Sorry for not using accent marks) the Status Notifier?
I see that you have Status Tray Plugin, too.
Good question 
I've just added the "Rendszerterhelés-figyelő" (picture) and moved it to the first place on the "systray" and i've changed nothing else (and Minimize all windows...).
And sometimes it loads only one instance after boot and sometimes (more frequently) loads (or display them) them with duplicates...
Try removing one of the above from the Items tab
Status Notifier or the Indicator bovitmeny object and test...
You can restart the panel easily with alt+F2 and enter in
xfce4-panel --restart
I removed "Indicator bovitmeny" and run the mentioned terminal command, but the panel disappeared and didn't launch and display... So now I have no panel... 
I know there is another command for the panel, but dont rememeber it now...
xfce4-panel
Or run the above in terminal if you need to just open panel preferences and restore the plugin.
Thanks, that did the tricks! I mean i have panel again!
And as i mentioned, i removed "Indicator bovitmeny" and now i only one Redshift and Wifi icon on the panel! Thanks for the help! I hope it remains still.
That is very odd. Removing a plugin should not crash the panel out and cause it to not restart... The xfce panel does not depend on any plugins to run...
Ah good. So it was status notifier and status tray plugin.
Glad you are fixed up.
Yeah something strange with panel in this version. I have another pc with Zorin OS 15.x and there is no problem at all with the panel...
After OS restart the Redshift still displayed duplicated. The Wifi is ok, theres only one icon on the panel. But i can live with that.
These warnings are actually normal... and can be ignored.
I won't go into boring detail on what each one means unless asked... and even then only when I feel like typing that much.
But I can get those same warnings if I run the xfce panel from terminal. They have been standing for many years.
I think this is a bug. It happens to me also my Bluetooth or ethernet icon duplicate sometimes