it has nothing to do with the Episerver, but actually prevents you to write code like following:

[ContentType]
public class SamplePage : PageData
{
    [LanguageSelection]
    [LanguageSelection]
    public virtual string SingleLanguage { get; set; }
}

because having multiple attributes on the same property does not make sense in this context.

To extend valdis's answer: that's a .NET framework thing and not Episerver specific, and it's to indicate that if you want to allow callers to apply your attributes multiple times or not.

One example is the lifecyle attribute, it should not be both Singleton AND Transient, so you would want it to be apply one time. But if you want a specific feature to be accessible only to people with two roles - they have to be both CommerceAdmins and EPiBetaUsers, for example, it's when you apply the Authorize attribute two times