I don't think so. "I have told you about it many times" is something different than "I have told you many times." In the latter, the "lo" (the "it" that the Spanish version refers to) is something that both parties understand to be what they're talking about. I think it's kind of similar to how Spanish speakers say, "Sí, lo sé" ("Yes, I know"), instead of just "Sí, sé" (although I don't think that's necessarily wrong). There's something that is "known," (e.g., where a restaurant is, or how stupid someone is) even though we don't acknowledge it in English with a word of its own (whereas Spanish does).
Duo told me I got this wrong. oh well, Duolingo doesn't always get it right. This is just the first part of a sentence: I've told you many times ... The second part could be: how much I love you; that you need to be careful; you really piss me off, etc. Without the second phrase the original sentence needs a direct object, so "I have told you -that- many times" would be the most natural/common English expression. "I have told -it- to you many times" might be used and would be understood by native English speakers.
DL accepted "I have said it to you many times". Homrul, your word order is a little awkward, but it may also be that DL is not recognizing your contraction "I've". Anyone- I was surprised that no one else had used "said" instead of "told". Isn't said a better translation of decir than told?