Since A.D. 1860 there's the Pontifical Yearbook (Annuario Pontificio) including a list of all legitimate Popes*.

There may have been debates whether some Antipopes should not be called Antipopes in the strict sense of the word. But there never was a serious debate whether there can be more than one reigning bishop of Rome at any time.

*) some say that recent Yearbooks are inaccurate, though

PS.  I apologize for continuing to call the book Annuncio Pontificio in my posts. It should be, as you correctly note, Annuario Pontificio.

St. Robert Bellarmine disagrees.

I think it begs the question even talking about excommunications during the so-called Western Schism
If a doubtful pope is no pope, then an excommunication from a doubtful pope is also doubtful, and not considered legitimate.
Which means if nobody was to blame for choosing the wrong pope, then nobody was to blame for ignoring the excommunications.

But in the GWS these claimants were in opposition to true popes. Isn't that the definition of an anti-pope?

I think so.

As the responses to this thread right from the very beginning clearly show, some sedevacantists have taken the term "dogmatic sedevacantism" to a whole new level. It's one thing even to say "I believe Pope Francis is not the Pope with the same certitude with which I believe in the Immaculate Conception, for the same authority assures me of both". It is another matter entirely to treat defection from the Catholic Faith, and apostasy into Protestantism or worldliness as a light matter, if SVism gets proved incorrect. 

If someone sincerely believes this and would do those things upon being proved incorrect, I'm not going to try to dissaude him from SVism, until he admits he would become an Indult Traditionalist, if Svism were proved false. SVism is easily proved false, btw.

There are two rock solid arguments against it, both 62 year SVism and single Pope Svism. But I won't go there till sedes get this right. 

I'll bet the Orthodox will have joined the Catholic Church, with the Consecration of Russia completed, in another 10 to 15 years. To "go Orthodox" is wrong and throws out many dogmas still taught in the New Catechism, including the Immaculate Conception and the Filioque. See this article on that: "whatever the past may have been, Pope Francis and Patriarch Bartholomew have invited all Christians to gather together in 2025 at Nicaea, to commemorate its 1,700th anniversary. Therefore, by God’s Grace, Greek and Latin Churches can once more profess dogma together ... 

Dear Orthodox Christians: A word from our hearts to yours — if we wish Christianity to successfully combat and entirely overcome the new paganism of the culture of death, of abortionism, contraception, divorce, pornography, and other forms of immorality and lawlessness, if we hope for the worldwide Church to receive more conversions from paganism and baptize more individuals into Christ and the Triune God, and make them members of the Church, the time to reunite is now and quickly.

The Immaculate Heart of Mary, the first defender of Christian civilization, alone warned the world about the dangers and errors of communism and the great persecutions threatening the Church and all Christendom. The history of the last century bears sad testimony to the truth of her words and the urgency of her calling. The time is ripe and the hour is now for the Greek and Russian Orthodox churches to profess the Filioque dogma and unite with the Catholic Church for the glory of God. 
From: https://onepeterfive.com/filioque-separated-east/

As the responses to this thread right from the very beginning clearly show, some sedevacantists have taken the term "dogmatic sedevacantism" to a whole new level. It's one thing even to say "I believe Pope Francis is not the Pope with the same certitude with which I believe in the Immaculate Conception, for the same authority assures me of both". It is another matter entirely to treat defection from the Catholic Faith, and apostasy into Protestantism or worldliness as a light matter, if SVism gets proved incorrect.

Your'e not really getting it, Xavier.  If you had asked the question, "What would you do if the dogma of the Immaculate Conception were proven false?" ... what would the response be?  It's a bad question because Catholics have certainty of faith regarding it.  So asking a Catholic a question along the lines of what IF a certain dogma were proven wrong is not legitimate ... since no Catholic would ever entertain your hypothetical.  Your'e doing the same thing here to the dogmatic sedevacantist.  But IF the Immaculate Conception were proven wrong, it would not be an inappropriate hypothetical response to say then that one would cease to believe in the claims of the Catholic Church.  If one cannot have certainty of faith regarding the Church's dogmatic teaching, then the entire Catholic system would be illegitimate.

What exactly do you think is meant by DOGMATIC sedevacantism anyway?

How can we be sure that only sedevacantists have taken this poll?

This can easily lead to non-sedevacantists taking the poll just to click on and promote their own current positions.

Susceptible to vote fraud.

We can't, especially given the tally currently shows 17 sedevacantists voted.  I didn't vote, so exactly who are these other 17 sedevacantists on this forum?

It would be interesting if those sedevacantists that voted posted who they are (no need to say how they voted, just that they voted).

Struthio is probably considered an example of a dogmatic sedevacantist. Yet in another thread when I asked him what if sufficiently many years passed, he answered:

"I would (have to) admit that my current assessment of the situation is or was wrong, as soon as the generation of the Robber Council will have passed away (which cannot happen while I have to continue in this life)."

Edit: Just saw, "UDS is a logical corollary of DS." Ok. I would have thought the Dogmatic SVist would at least become non-dogmatic Svist in light of powerful evidence to the contrary. Also, a dogma can only be believed with infallible faith on the authority of the teaching Church. Otherwise, it would remain something like at most a theological conclusion only. But the Church has not ruled that Pope Francis is not the Pope. Ergo, there cannot be de fide certainty that such is the case. 

An analogy would be someone writing AGAINST the Immaculate Conception the day before it was defined. He may think he has good theological arguments for the same. But if it were dogmatically defined by the Church, and he wanted to remain Catholic, he would have to say, "I retract my opinion and submit to the infallible judgment of the teaching Church".

In the same way, if it were proved, for e.g. (1) that SVism leads to EVism, and (2) EVism is heretical, a sedevacantist who intends to remain Catholic should retract what was never a dogma of faith, returning to the Authority of the Church that he thought had defected.

An analogy would be someone writing AGAINST the Immaculate Conception the day before it was defined. He may think he has good theological arguments for the same. But if it were dogmatically defined by the Church, and he wanted to remain Catholic, he would have to say, "I retract my opinion and submit to the infallible judgment of the teaching Church".

This is probably a good analogy to describe those who are in between moderate and ACTUAL dogmatic sedevacantism.  It's a personal opinion that this is dogma, but it's not yet backed by the authority of the Church, so its denial cannot be formal heresy.

I'll give you an example with myself.  I am convinced that "Rewarder God" theory (in the EENS category) is objectively heretical.  But since the Church has not definitively condemned the opinion as heretical, I cannot hold anyone who believes in it to be a formal heretic.  I believe it to be heretical and have my arguments for why I believe this, but if I were a priest, for instance, I would not refuse Communion to someone who held that opinion because I don't have that authority.  This is in fact my big beef with the Dimond brothers.  I feel that it entails a schismatic mentality to exclude from the Church those whom the Church has not excluded.  It's one thing to argue that the position is objectively heretical, and quite another to consider people who hold it to be non-Catholics ... since it hasn't been so defined by the Church.  They make syllogisms from other dogma to prove that the conclusions are heretical, but these syllogisms involve human reasoning and therefore do not rise to the level binding consciences.

So, for instance, if someone had argued before the definition of the Immaculate Conception that this was a revealed dogma and its denial was objectively heretical, he would in fact be right, since it's always been objectively a dogma.  But for him to then go on and refuse Communion to someone who didn't believe in it before the definition, that would have been schismatic.