In the southern U. S., “y’all” is second person plural, period. To say that it can be singular is simply incorrect. Other regional dialects of American English make no such distinction between second person singular and plural (except perhaps for “you’uns” or “youse”, which appear to be fading from use, or are actively being suppressed, and where their apparent origin as plural forms may have been forgotten – which misunderstanding is extended, in ignorance, to “y’all”).
The impression that “y’all” can ever have a singular meaning may come from misunderstanding the common hospitable southern expression, “Y’all come back, y’hear?” because it can indeed sometimes be spoken to only one person. However, the implication is always that both the addressee AND his or her family, friend etc. who are with him or her are meant (though the others may already be out in the car, waiting to go…). All of those who are (or were) with the person addressed are included, whether they are still within earshot or not. There are no doubt other instances in which one person is addressed with “y’all” – but the reference is always to more than one (“Why don’t y’all come over this weekend?” means “Why don’t you and your husband, wife, etc. come over…?” even though the person addressed may be alone (except for the speaker) at the time.
So, “y’all” is always plural. The only correct usage of the expression “all y’all” I can imagine would be for clarification, confirmation or emphasis (e.g., “your whole family – that’s right, I mean all 12 of you”). Otherwise, it is simply evidence of misunderstanding.
“Ya’all” may be an acceptable alternate spelling, and “you all” (of which “y’all” is a contraction) is sometimes still heard in parts of the South as well, though not much around here (Houston). Same meaning – plural. The plural possessive ("y’all’s) is also used – not as much – and may not be regarded by some as equally acceptable to "y’all itself.
(This is from someone not born in the South – who was in fact stigmatized in the fourth grade (in Tennessee) by his midwestern accent and speech mannerisms, and learned southern as a second language, but now speaks it with reasonable fluency, and by choice…)
So, remember, y’all – “y’all” is ALWAYS plural…