Doesn’t always happen, but most of the time I boot into the OS, there are two functional network applets in the tray. Any ideas how to fix?
I’ve noticed the same issue. There appears to be two applets displaying the network icon in the panel/tray. There is one network icon in the “Indicator Applet Complete” applet, and one in the “Notification Area” applet. I noticed this by moving stuff around in the panel. I guess one could remove the “Notification Area” applet to “fix” it, but that’s an ugly solution. 
I’m not sure if both applets are supposed to be running, but it’s a little more info regarding the issue for the developer folks.
Never faced this type of issue. By any chance, did you modify panels/layouts using MATE Tweak or in some other way excessively? Because that might be the cause. Try resetting the panel, that should fix things…
Same issue here, so I use the following command to restart the panel. I set F11 to that command.
killall mate-panel
It also happened to me the one or other time. It disappeared after a system restart.
Confirmed still a bug in Ubuntu Mate 18.04.1 LTS. Restarts, resetting the panel, and switching panel layouts do not fix the issue. One icon is from “Notification Area” and the other is from “Indicator Applet Complete.”
I'm seeing this as well, attaching screenshot if it helps.
Killing the panels doesn't get rid of it, nor does reseting it.
Also confirming what paulr said:
There is one network icon in the “Indicator Applet Complete” applet, and one in the “Notification Area” applet.
I have find a solution . If I add 2 seconds delay to nm-applet only 1 nm-applet start 
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