With verbs expressing doubt, even when they are in negative, the subjunctive is needed. Here is a lesson about this:
[competency id= "6235"]
I've just double checked the use of subjunctive with "dudar" in negative, and there is a nuance here when we use "no dudar que". In this case, the meaning of this verb in negative is "to hope/to trust". This is why in this specific case it is used with subjunctive. So, this sentence means: "Yo espero/confío en que nosotros nos queramos siempre."
We will add an extra note to the lesson to clarify.
I hope this clarifies your doubt.
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