Subform Client Script

Subform Client Script

Good day, 

I have a subform where users can subscribe to various magazines. I would like to prevent the user from selecting the same magazine twice when adding a new row. Is there a way to prevent the user from doing this? (Can it be done via a client script?)

I also have two date fields in the same subform, where the user can select a start date and end date for the subscriptions. How can I alert the user if the start date is after the end date? 

Lastly, I am unable to get the values in a subform when I am using a Client Script. Even though the trigger is set to "onChange" of  a subform field, it does not look like a change is registered...

Any help with subform validations and alerts would be greatly appreciated.
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