Yes, you're right. But "persona regular" does exist, and is also in use. (But my guess is it is only used as a law term, in law texts)
In this case, Duolingo is wrong! We have to report it!
As you said: "A regular person" = "una persona normal", "una persona común", "corriente".
Just several questions before this one it asked the exact same statement be translated from English to Spanish. When I wrote using the word regular after the word persona it came back and said I was incorrect. I have seen numerous places where Duolingo actually contradicts itself.
In addition, in the U.S., "regular" is also a medical term. Some people have trouble with "being regular."
"Regular" also has other meanings -- I don't know if they correspond with the Spanish. http://www.dictionary.com/browse/regular
It's more common for a Spanish speaker to say "Soy una persona normal" referring to being normal. "Soy una persona común" is the closest to saying "I am a regular person" if regular refers to common. "Soy una persona regular" is... kind of awkward. It probably means that the person is neither good nor bad. Hope it helps.
Yes, "regular" = de poca calidad.
So, you can insult a person saying "eres regular"? (It looks like rather like a medical term for a woman)
Yes. It's the way I understand it.
Madonna said (in my example) "I'm just a regular person" = not a celebrity, just a normal person.
What do you think the translation for "not a public figure" should be in Spanish, in this case, for this kind of "regular" meaning?
Apparently, while "I am a normal person" and "I am a regular person are correct; this is closer in meaning to "I am just an ordinary person." or more idiomatically "I am just an average Joe." According to my Spanish speaking friends, regular means "ordinary" in a not so flattering way, and can actually be used as an insult.
The speech version of this is horrible quality. The guy says, "Soy una persona pergubulrrrr" (or something very mumbled like that). I was able to infer it was "regular" but if it was the first question in this lesson I would have never figured it out. Where do they get their sound samples or speech robots?