Description of problem:
We have two services sharing the same external IP (10.10.10.10):
$ oc get svc
NAME        TYPE      CLUSTER-IP    EXTERNAL-IP              PORT(S)                                                                                              AGE
uperf-service-tcp   LoadBalancer   172.30.5.230     10.10.10.10                            30000:31205/TCP   4m41s           
uperf-service-udp   LoadBalancer   172.30.175.221   10.10.10.10                            30000:30090/UDP   4m41s

Upon editing the first of them to change the externalTrafficPolicy definition, its external IP changes (10.10.10.11):
$ oc get svc
NAME        TYPE      CLUSTER-IP    EXTERNAL-IP              PORT(S)                                                                                              AGE
uperf-service-tcp   LoadBalancer   172.30.5.230     10.10.10.11                            30000:31205/TCP   4m41s           
uperf-service-udp   LoadBalancer   172.30.175.221   10.10.10.10                            30000:30090/UDP   4m41s

Version-Release number of selected component (if applicable):
OCP version: 4.10.0-rc.1
MetalLB version: 4.10.0-202201310820

How reproducible:
100%

Steps to Reproduce:
1. Create two services sharing an external IP
2. Update the externalTrafficPolicy definition of one of them

Actual results:
The external IP of the service changes

Expected results:
In order of preference:
1. The service gets updated and the external IP of the service does not change
2. If (1) it is not possible due to some implementation limitation, then the update of the service should fail, stating that is not possible to update the ETP of a service that is sharing its external IP. Imho it is better to reject the update than to allow it and change the external IP without warning.

Additional info:
It looks like the controller has properly identified the situation:
{"caller":"level.go:63","error":"can't change sharing key for \"default/uperf-service-tcp\", address also in use by default/uperf-service-udp","event":"clearAssignment","level":"info","msg":"current IP not allowed by config, clearing","service":"default/uperf-service-tcp","ts":"2022-02-16T19:11:06.262179646Z"}