Translation:I had to go see a doctor yesterday.
"go see ..." is American English; British (and New Zealand) English would require "go to see ..."
Several English variants are possible: 1) go to see, 2) go see, 3) go and see, 4) see, 5) go to. There is nothing to be gained by pushing BrE vs. AmE. Since Duolingo is an American company, don't expect BrE to be preferred if there are differences. Every effort is made to accept all reasonable translations.
The vast majority of English speakers speak American English: https://en.wikipedia.org/wiki/English-speaking_world
Did you report it as another alternative?
https://www.duolingo.com/skill/fr/Verbs%3A-Past-Imperfect/tips-and-notes This link doesn't work, but just click on the Past Imperfect skill and click on the lightbulb when you are on the web version of Duolingo.
In addition 2 months later:
Even though you have the imperfect in French, the English translation is the same as for the “J’ai dû aller voir...” and this is a difference in English because you cannot say “I was having to go see...”. “Have” is a stative verb which is not used in this way.
It is not “going” that is in imperfect but “having to go”, so this verb takes on a different meaning. The imperfect version of “I had to go see...” means “I was supposed to go see...” while the passé composé version of “I had to go see...” means “I must have gone to see...”
2019-02-11 So you're saying that this sentence means someone made an appointment for me that I was compelled to keep, not that I suffered an acute condition that required medical treatment? I would be more likely to use it in the 2nd case, which would not be imperfect.
You can have to go see a doctor for different reasons. I did not say that. The imperfect implies that it is not a sudden reason, but one that I knew about. Who says someone else made the appointment? I could have made the appointment myself. Perhaps I need to go for a follow-up appointment, or to a specialist, or just my annual doctor’s appointment, but it was something that I knew that I was going to have to do during the past and then I did, but the imperfect expresses that there was a time during which I knew I was supposed to do this. The passé composé would be used for something sudden or that happened and was done. The passé composé also covers our present perfect tense.
I do not know whether the combined populations of all non US English speaking countries exceed the US population (some of whom are not native English speakers. However the point is that a considerable number (if not the majority) of the world's native English speakers would use "go to see" and as such this should be accepted by Duolingo. Why is Duolingo trying to undermine their excellent work by this obstinacy?