¡Hola a todos!
The topic of direct and indirect object pronouns is indeed always a tricky one, since Spanish just works so differently from English!
To say “(We) are going to see her,” there are three main options in Spanish:
La vamos a ver.
Vamos a verla.
La vamos a ver a ella.
In the first and second options, we're using the direct object pronoun la “her” in each of its possible positions in this sentence: before the conjugated verb (La vamos a ver) and attached to the end of the infinitive (Vamos a verla). It isn't actually possible for us to repeat la “her” twice in one sentence and say "La vamos a verla" - Spanish doesn't allow this. We can only use la once, and we have to choose one of the possible positions for direct object pronouns.
There is also the third option, though, which I think may be the one that you had trouble with in the flashcards: La vamos a ver a ella. In this sentence, we're using both the direct object pronoun la “her” and the prepositional pronoun ella “her” to refer to the same person - which is both perfectly correct and very common in Spanish, and can seem just bizarre to native English speakers! The reason why Spanish speakers use these two pronouns in one sentence is for clarity or for emphasis - essentially, it's like saying “We are going to see her, specifically.” It might help to revisit the section on this in Lesson 5.6 titled “Using Prepositional Pronouns With A,” where there's a full explanation on how this works.
Now, because there can be so many ways of saying the same thing in Spanish, this can indeed make things tricky when you're doing the Reinforcement activities! That's why we provide hints through the literal translations - you'll notice that literal translations are always included for sentences where pronouns are attached to the ends of infinitives (like in the sentence Quiero enviarla “(I) want to send it” (literally: “… to send+it”)), where pronouns are attached to the ends of present participles (like in the sentence Estoy comprándolos “(I) am buying them” (literally: “… buying+them”)) and where both direct object and prepositional pronouns are used in one sentence (like in the sentence La vamos a ver a ella “(We) are going to see her” (literally: “We her are going to see her”)). So these little hints can help you see which version the activity has in mind. And if you happen to go through an activity and realize that the answer you gave is just another correct way of saying the sentence, you can always choose to give that question an “easy” rating so that you don't get quizzed on it again.
I hope that this is helpful! These kinds of pronouns are indeed complicated, and as Scott_C says, sometimes simply moving on to the next lesson can help if you're feeling frustrated - it can give you a bit of a brain break, and you will see more examples that can help the principles start to click. Or, you can always come on here with your questions, and we'll help you out!
Do let me know if any of this is unclear or if you'd like more explanation on anything!
Saludos,
Liss