How about creating a custom validator for this? You could either give a warning or an error message to the editors.

http://henrikm.com/custom-episerver-property-validation/

Yes, I tried it and it works perfectly!!

Thanks a lot, this made my day :)

A minor thing... would be great to be able to present the actual proprerty display name to the editor (and not the actual property name)...

Great! 
Regarding the display name, you could check out Grzegorz answer in this forum thread: http://world.episerver.com/forum/developer-forum/Developer-to-developer/Thread-Container/2015/8/localized-property-name-in-validation/

Yes I found his answer and borrowed it :-)

Here is the code if anyone is interested, not many rows

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
	public sealed class DisableDuplicates : ValidationAttribute
	{
		protected override ValidationResult IsValid(object value, ValidationContext validationContext)
		{
			var propertyDefinitionRepository = EPiServer.ServiceLocation.ServiceLocator.Current.GetInstance<IPropertyDefinitionRepository>();
			var propertyDefinitionId = ((PageData)validationContext.ObjectInstance).Property[validationContext.MemberName].PropertyDefinitionID;
			var propertyDefinition = propertyDefinitionRepository.Load(propertyDefinitionId);
			propertyDefinition.LocalizationService = LocalizationService.Current;

			if (IsDuplicate(value as ContentArea))
			{
				return new ValidationResult(
					$"You can not have duplicate content in: '{propertyDefinition.TranslateDisplayName()}'",
					new[] { validationContext.MemberName });
			}

			return ValidationResult.Success;
		}

		bool IsDuplicate(ContentArea area)
		{
			if (area == null || area.IsEmpty)
				return false;

			if (area.Items == null || area.Items.Any() == false)
				return false;

			List<int> items = new List<int>();

			foreach (var cai in area.Items)
			{
				if (items.Contains(cai.ContentLink.ID))
					return true;

				items.Add(cai.ContentLink.ID);
			}

			return false;
		}
	}

Thanks for sharing! :-)

Sounds like a good solution. I might steal it :)

You could also simplify IsDuplicate method to single line expression :)

private bool IsDuplicate(ContentArea area)
{
    return area?.Items.DistinctBy(c => c.ContentLink.ToReferenceWithoutVersion()).Count() != area?.Items.Count;
}

Excellent!