Doc says:

/// Throws when multiple attributes are found (the attribute must allow multiple).

Implementation:

        Attribute[] cachedAttributes = GetCustomAttributesCached(attributeProvider, inherit);

        foreach (Attribute cachedAttribute in cachedAttributes)
        {
            if (cachedAttribute is TAttribute cachedAttributeAsTAttribute)
            {
                return cachedAttributeAsTAttribute;
            }
        }

        return null;

This method is called for the following attributes:

• AsyncStateMachineAttribute
  • Already doesn't allow multiple. So it's okay.
• TimeoutAttribute
  • Already doesn't allow multiple. So it's okay.
• PriorityAttribute
  • Already doesn't allow multiple. So it's okay.
• ClassInitializeAttribute
  • Already doesn't allow multiple. So it's okay.
• ClassCleanupAttribute
  • Already doesn't allow multiple. So it's okay.
• TestClassAttribute
  • We should throw if multiple are found. We cannot force it to not allow multiple because it's not sealed.
• TestMethodAttribute
  • We should throw if multiple are found. We cannot force it to not allow multiple because it's not sealed.