Custom List - Validation

Hi,

I am using the SharePoint App "Custom List" to store some Data. My custom list has three columns

- Title

- Keywords (multiple comma separated keywords)

- Url

If I create and save a new item I want to perform a validation. If there is already an item in the list which contains the same keyword as entered in the form, a validation massage should appear and the saving should be cancelled.

Is it possible to achieve this with a validation formula and how it should look like? What would be the best approach to implement such a validation?

Thanks,

Nick