* If you mean consonants that never appear twice in a row in a word: There are no such consonants in the English language. You can find examples of double letters for almost every consonant, even those that are usually singular like "l" (e.g., "call") and "m" (e.g., "mammal").
* If you mean consonants that are never doubled for spelling purposes: There are a few consonants that are rarely doubled in English spelling:
* C: While you can find "cc" in words like "accede" and "occasion", it's not a common doubling.
* Q: The letter "q" is almost always followed by "u", making "qq" a very rare occurrence.
* X: The letter "x" is rarely doubled in English spelling. It can appear doubled in words like "oxox" (a rare word for a type of fruit) but is generally uncommon.
It's important to note that even these consonants can be doubled in specific cases, but it's not a regular occurrence.
The reasons for these exceptions are often based on the historical development of the English language, and the specific sounds that these letters represent.