Somehow I never learned to double tongue. Actually, I know how this happened. I had, and still have, a very fast single tongue, so for most of the times when one might double tongue, I single tongued.
For the non-brass playing readers, brass players start notes with a "ta," "tu," or "da," using the tongue against the top front teeth. When the speed of the notes is too fast to keep up by single tonguing "tu-tu-tu-tu," brass players switch to double tonguing, "tu-ku-tu-ku" etc., using the tongue for the tu, then pulling it back and saying ku for the next note. It's a very useful technique.
My teacher in college, Milan Yancich, assigned me double tonguing exercises, which I did half-heartedly. So I continued through grad school and Civic Orchestra single tonguing, occasionally faking a passage that was too fast for me to single tongue.
I have learned along the way that I'm not alone. A number of musicians have told me how difficult they find double tonguing. And I recently read that Rafael Mendez did not need to double tongue, because his single tonguing was incredibly fast. Mine is not that fast.
Fast forward several decades and I find myself playing first horn in an excellent band in my community. Several pieces on the program this past fall demanded double tonguing, including the Maslanka Symphony #4. The time for faking was over. So I pulled out the Arban book and started working on double tonguing, very slowly, every day. Progress was slow. If I tried to increase the tempo of the exercises too much, they would crash and burn. I thought I would never master even one page - Arban page 175. I really didn't know if I was going to be able to play the passages that needed double tonguing by the band concert date. There's the popular theory now that a person can become an expert by spending 10,000 hours of focused practicing. I have certainly spent more than that much on horn playing overall, and I really hope I don't need to spend 10,000 hours just on double tonguing. But focused practice is the answer, whatever one is trying to learn.
I did get better. I was able to use double tonguing on the concert, though not at full volume. For me, the experience of trying to learn something new that required both physical and mental effort was very fulfilling. Though I learn new things all the time, this particular technique was challenging in a way that most things in my life are not. It required concentrated effort, patience, and weeks and weeks of practice. And no, I have not mastered it yet.