"A veces lo veo a él en la calle."
Translation:I sometimes see him on the street.
I agree, 'a el' is claryifying, so is optional. 'lo' is the pronoun and is required... in fact, using 'lo' without 'a el' (which I believe is what a native speaker would do) is more concise than amcm96's version, and the ambiguity is probably negligable within the context of a conversation.
It seems that some of the sentences we've been constructing on Duolingo up to this point should have been using these object pronouns so perhaps we need to refine our understanding now :)
Thanks for the feedback. Another question for you, could i also say "A veces le veo a el en la calle" to mean the same thing? i.e. I'm not clear when we use "lo" and when we use "le". I had thought if we're talking about people we use the "le" form and if we're talking about things and abstractions we use the "lo" form. But this example "a veces lo veo a el en la calle" shoots down my theory.
Lo and la are used for direct objects, and le is used for indirect objects. In the English sentence "I wrote him a letter," the letter is the direct object because it's what you wrote, and "him" is the indirect object because he's what you wrote -to-.
There are exceptions to this. In some parts of Spain they use "le" instead of "lo" as the direct masculine pronoun but the global standard is "lo."