Plasma 5.19.5 running on Ubuntu 20.10
When rearranging the icons of running applications on the panel in taskbar mode, sometimes it creates a widget on the panel.

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inxi -F
System:    Host: shmuel-ubuntu Kernel: 5.8.0-44-generic x86_64 bits: 64
           Desktop: KDE Plasma 5.19.5 Distro: Ubuntu 20.10 (Groovy Gorilla)
Machine:   Type: Desktop Mobo: ASUSTeK model: PRIME B365M-A v: Rev X.0x
           serial: <superuser/root required> UEFI: American Megatrends v: 1203
           date: 10/10/2019
CPU:       Info: 8-Core model: Intel Core i7-9700 bits: 64 type: MCP L2 cache: 12.0 MiB
           Speed: 4588 MHz min/max: 800/4700 MHz Core speeds (MHz): 1: 2575 2: 1965
           3: 1111 4: 3564 5: 1420 6: 2365 7: 1476 8: 1063
Graphics:  Device-1: Intel UHD Graphics 630 driver: i915 v: kernel
           Device-2: IMC Networks XHC Camera type: USB driver: uvcvideo
           Display: x11 server: X.Org 1.20.9 driver: modesetting unloaded: fbdev,vesa
           resolution: 1: 1920x1080~60Hz 2: 1920x1080~60Hz
           OpenGL: renderer: Mesa Intel UHD Graphics 630 (CFL GT2) v: 4.6 Mesa 20.2.6
Audio:     Device-1: Intel 200 Series PCH HD Audio driver: snd_hda_intel
           Device-2: JMTek LLC. type: USB driver: hid-generic,snd-usb-audio,usbhid
           Sound Server: ALSA v: k5.8.0-44-generic
Network:   Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet
           driver: r8169
           IF: enp3s0 state: up speed: 1000 Mbps duplex: full mac: a8:5e:45:e2:87:3c
Drives:    Local Storage: total: 2.26 TiB used: 693.45 GiB (30.0%)
           ID-1: /dev/sda vendor: Kingston model: SA400S37480G size: 447.13 GiB
           ID-2: /dev/sdb vendor: Toshiba model: HDWD120 size: 1.82 TiB
Partition: ID-1: / size: 139.05 GiB used: 36.84 GiB (26.5%) fs: ext4 dev: /dev/sda6
Swap:      ID-1: swap-1 type: file size: 8.00 GiB used: 95.0 MiB (1.2%) file: /swapfile
Sensors:   System Temperatures: cpu: 29.8 C mobo: 27.8 C
           Fan Speeds (RPM): N/A
Info:      Processes: 344 Uptime: 8h 26m Memory: 15.50 GiB used: 8.50 GiB (54.9%)
           Shell: Bash inxi: 3.1.07

What widget is created? Please be a tad more specific, especially which particular taskbar widget you are using and what your settings are. Just tried here, using the Icons only taskbar and I see no problems.

To be more specific: I have Google Chrome running. I slide its icon to the left on the panel, passing the icons of other running programs such as Firefox and VirtualBox. A Chrome widget now appears on the panel. So I now have two Chrome icons on the panel: one that is pinned, and a new one that is in the form of "Add to Panel (Widget)".
After a little experimenting, I think this happens when I move the icon fast. When I move it slowly and gently, it does not create a second icon. Not sure if this is coincidental or not.