User can create same domain mapping multiple times

Description of problem:

While creating the domain mapping, user can create the same domain mapping multiple times.

Prerequisites (if any, like setup, operators/versions):

Install OpenShift Serverless Operator and create Knative Eventing CR

Steps to Reproduce

1. Select any import flow
2. Select Serverless Deployment resource type
3. Click on Advanced Option
4. Create a domain mapping "domain.org"
5. Type the same domain mapping again and it will get created

Actual results:

Same domain mapping get created multiple times

Expected results:

It shouldn't create the same domain mappings multiple times, instead, it should ask the user to select an already created one when he tries to create the same again.

Reproducibility (Always/Intermittent/Only Once):

Always

Build Details:

4.9.0-0.ci-2021-07-28-013104

Additional info: