This example (a word with a letter that repeats in a different case, and no letters that repeat in the same case) would have been very helpful to have! It implicitly clarifies the statement. However I was very confused when I received your reply until I noticed that kata descriptions and test cases can also differ depending on the language it's viewed in. In my case this example is missing from the C++ description (and most other languages in fact), whereas it's present in the Javascript version.

C++:

isIsogram "Dermatoglyphics" == true
isIsogram "moose" == false
isIsogram "aba" == false

Javascript:

isIsogram("Dermatoglyphics") == true
isIsogram("aba") == false
isIsogram("moOse") == false // -- ignore letter case

Consistency is important! However my advice to other warriors reading this would be to check the other language test cases for hints, just in case. :)

"Ignore letter case." is a vague, and possibly contradictory, statement. Please clarify to something akin to "Assume upper and lower-case letters count as the same letter."

If you have to handle letter casing to reach the correct solution then you're not ignoring them, at least from a certain context.