Deleted "duplicate" connections persist in the list of network connections

When I delete a connection from the Network Manager, nothing seems to happen. If I attempt to delete it again, I get an error,

No such interface 'org.freedesktop.NetworkManager.Settings.Connection' on object at path /org/freedesktop/NetworkManager/Settings/18

To reproduce, start by finding a connection that ends in "1" or "2". It's unclear to me how these connections were created to begin with, but they appear to be duplicates of existing connections - e.g. I create a connection called "N5X" several months ago, and now I had "N5X 1" and "N5X 2". Perhaps changing passwords triggered the creation of these "duplicate" connections?

Anyway, when the first connection I tried to delete was not a duplicate one, deletion went OK. When I started by trying to delete a duplicate connection, and subsequent connection I tried to delete generated the error above.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: network-manager-gnome 1.2.6-0ubuntu0.16.04.4
ProcVersionSignature: Ubuntu 4.13.0-32.35~16.04.1-generic 4.13.13
Uname: Linux 4.13.0-32-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Feb 18 16:11:43 2018
EcryptfsInUse: Yes
ExecutablePath: /usr/bin/nm-connection-editor
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
InstallationDate: Installed on 2017-10-06 (135 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
NetworkManager.state:
 [main]
 NetworkingEnabled=true
 WirelessEnabled=true
 WWANEnabled=false
SourcePackage: network-manager-applet
UpgradeStatus: No upgrade log present (probably fresh install)
nmcli-dev:
 DEVICE TYPE STATE DBUS-PATH CONNECTION CON-UUID CON-PATH
 tun0 tun connected /org/freedesktop/NetworkManager/Devices/4 tun0 14c5e3dd-ce90-44d8-ae08-30f072970242 /org/freedesktop/NetworkManager/ActiveConnection/7
 wlp2s0 wifi connected /org/freedesktop/NetworkManager/Devices/1 N5X 7a73df40-4223-4970-a9e5-7a36c99ec793 /org/freedesktop/NetworkManager/ActiveConnection/8
 lo loopback unmanaged /org/freedesktop/NetworkManager/Devices/2 -- -- --
nmcli-nm:
 RUNNING VERSION STATE STARTUP CONNECTIVITY NETWORKING WIFI-HW WIFI WWAN-HW WWAN
 running 1.2.6 connected started full enabled enabled enabled enabled disabled
no_proxy: localhost,127.0.0.0/8,::1