I found a relevant section
in Karl Gottlob Zumpt's A Grammar of the Latin Language
Zumpt wrote:With most of these compound verbs, the preposition (or an equivalent one) may be repeated with its proper case. This is most frequently done in prose with the verbs compounded with ad, con, de, in; as adhibeo, confero, conjungo, communico, comparo, contendo, imprimo, inesse, inscribo, interesse (in the sense of, to be between, or, there is a difference); e.gr. Conferte hanc pacem cum illo bello; consilia sua cum aliquo communicare; in hac vita nihil inest nisi miseria. Incumbo is used with a dative, in the literal sense of leaning or pressing upon; incumbere baculo; in the sense of applying to a pursuit, with in or ad; ad laudem incumbere; in rempublicam incumbere.
Note: These are the general principles of the difficult doctrine concerning the syntax of compounded verbs; it must be pursued further by the help of the lexicon, and attention to the usage of the best authors. It may be further remarked, that many verbs compounded with ab, de and ex, either take the ablative or repeat the preposition; abesse, absistere, abstinere, abire, exire, decedere, excedere, dejicere, depellere, efferre, evadere.
I am merely a newbie, but to me the second paragraph sounds as if there is no real general rule telling us whether to use the same preposition with a compound verb or not. Adrianus' speculation about the repetition acting as some sort of intensifier is interesting. But it does not seem to work with the other prepositions, e.g. adhibere (Perseus-Link)
Should anyone know of a working general rule, I am all ears. I find the proper use of preposition + compound verb rather perplexing.
A good point made in a different Latin forum
And, yes, in later periods there was much redundancy. However, it’s not really any worse than ‘it is enclosed within the sack’, and arguably less redundant than ‘it is enclosed inside the envelope’, at which we don’t bat an eyelid.