P1M has hit the nail right on the thumb.

Although there are several spellings, only one is correct. If you are going to refer to the knife in the Gaelic, it is spelled sgian dubh.

The problem with relying on phonetic spelling is we are not properly teaching those that follow. What happens when we have an entire generation thinking that kat, dawg and pleeze are spelled correctly? It seems we already have a generation or two that think the words their, they're and there are completely and seamlessly interchangeable.