To keep the same emphasis, that would be ihr seid alle Menschen.
The Duolingo translation is because a recent change to how practice sentences are created keeps insisting on ihr = "you all". I don't think there's an especially good-sounding way to translate this accurately into English, but simply "you are humans" should suffice.
Ich = I Du = Thou Er, sie, es = He, She, It Wir = We Ihr = Ye Sie = They Sie = You »formal«
That was actually how early English personal pronouns were; it was still quite similar with other conservative German languages such as German »Hochdeutsch« and Icelandic »Íslenska«, and, of course, Ænglisc was the medieval germanic language spoken by the Jutes, Angles and Saxons with dialectal variations on what is today England and Great Britain.
What's the difference between the word (Ihr) and (Ihre)?
ihr (lowercase) as a personal pronoun in the nominative case means "you", referring to several people: ihr seid Frauen "you are women"
ihr (lowercase) as a personal pronoun in the dative case means "(to) her": ich gebe ihr das Geld "I give her the money"
ihr (lowercase) as a possessive determiner (before a masculine or neuter noun) means "her" or "their": ihr Auto "her car / their car"
ihre (lowercase) as a possessive determiner (before a feminine or plural noun) means "her" or "their": ihre Kinder "her children / their children"
Ihr (uppercase) as a possessive determiner (before a masculine or neuter noun) means "your": Ihr Name "your name"
Ihre (uppercase) as a possessive determiner (before a feminine or plural noun) means "your": Ihre Telefonnummer "your telephone number"
At the beginning of a sentence (where the first word is always capitalised), you can't tell the difference between ihr and Ihr nor between ihre and Ihre. So in full sentence, Ihre Mutter singt. can mean any of "her mother is singing / their mother is singing / your mother is singing".
For possessive determiners (before a noun), the choice of no ending versus -e ending depends on the gender of the following word.
For personal pronouns (that stand alone, not before a noun), this is not a consideration.
Duo's sentence here, Ihr seid Menschen., starts with ihr -- it's only capitalised because it's the first word of a sentence. It's the subject of seid and and so it's in the nominative case and must mean "you".
I don't think Ihre can ever mean "you".
And finally, just because it's before a noun doesn't necessarily mean that it's a possessive determiner; before an uncountable noun or a plural noun (which doesn't need a determiner), it might just be a personal pronoun.
In this way, ich kaufe ihr Bier is exactly as ambiguous as "I buy her beer" is in English -- is it the equivalent of "I buy him beer" (personal pronoun + noun) or "I buy his beer" (possessive determiner + noun)?