the following reproduces both on rhel-9.2 (5.14.0-244.el9.x86_64) and Fedora 37 (6.1.7-200.fc37.x86_64):

Script:
```
#!/bin/bash

set -ex

ip netns del x &>/dev/null || :
ip netns add x

_ip() {
ip -netns x "$@" || :
}

_ip link add net1 type dummy
_ip link set net1 up

_ip route append 7.7.7.0/24 dev net1
_ip -4 route append local default dev net1 table 10223
_ip -4 route append 192.168.4.0/24 table 10223 nexthop via 7.7.7.1 dev net1
_ip -4 route prepend default dev net1 proto kernel table 10223

_ip -d -4 route show table all

_ip -4 route append 192.168.4.0/24 table 10223 nexthop via 7.7.7.1 dev net1

echo ">>>>>>"
_ip -d -4 route show table all
```

Output:
```
+ ip netns del x
+ ip netns add x
+ _ip link add net1 type dummy
+ ip -netns x link add net1 type dummy
+ _ip link set net1 up
+ ip -netns x link set net1 up
+ _ip route append 7.7.7.0/24 dev net1
+ ip -netns x route append 7.7.7.0/24 dev net1
+ _ip -4 route append local default dev net1 table 10223
+ ip -netns x -4 route append local default dev net1 table 10223
+ _ip -4 route append 192.168.4.0/24 table 10223 nexthop via 7.7.7.1 dev net1
+ ip -netns x -4 route append 192.168.4.0/24 table 10223 nexthop via 7.7.7.1 dev net1
+ _ip -4 route prepend default dev net1 proto kernel table 10223
+ ip -netns x -4 route prepend default dev net1 proto kernel table 10223
+ _ip -d -4 route show table all
+ ip -netns x -d -4 route show table all
unicast default dev net1 table 10223 proto kernel scope link
local default dev net1 table 10223 proto boot scope host
unicast 192.168.4.0/24 via 7.7.7.1 dev net1 table 10223 proto boot scope global
unicast 7.7.7.0/24 dev net1 table main proto boot scope link
+ _ip -4 route append 192.168.4.0/24 table 10223 nexthop via 7.7.7.1 dev net1
+ ip -netns x -4 route append 192.168.4.0/24 table 10223 nexthop via 7.7.7.1 dev net1
+ echo '>>>>>>'
>>>>>>
+ _ip -d -4 route show table all
+ ip -netns x -d -4 route show table all
unicast default dev net1 table 10223 proto kernel scope link
local default dev net1 table 10223 proto boot scope host
unicast 192.168.4.0/24 via 7.7.7.1 dev net1 table 10223 proto boot scope global
unicast 192.168.4.0/24 via 7.7.7.1 dev net1 table 10223 proto boot scope global
unicast 7.7.7.0/24 dev net1 table main proto boot scope link
```

Note that after the script runs, the route

unicast 192.168.4.0/24 via 7.7.7.1 dev net1 table 10223 proto boot scope global

is twice in the output.

This causes a problem, because NetworkManager wants to put routes in a dictionary/cache, and it cannot cope with having the same route twice. Meaning, in the cache this route can only be once, so we delete this route once with

ip -netns x route delete unicast 192.168.4.0/24 via 7.7.7.1 dev net1 table 10223 proto boot scope global

NetworkManager will receive one RTM_DELROUTE event and remove the route from the cache (although, there this route still exists – once).