Retired Bishop of Corpus Christi suspects Benedict XVI is still Pope.

Only God knows whether or not Francis is an antipope, writes retired Bishop René Gracida, 94, of Corpus Christi, Texas. In two posts (September 5 and 29) on his blog “abyssum.org” he expresses doubts whether Francis' election was "valid and licit”.

According to Gracida the Apostolic Constitution Universi Dominici Gregis excommunicates any cardinal who participates in a conspiracy for the election of a new pope, “There is no doubt that Francis was party to a conspiracy to get him elected.” For Gracida an excommunicated cardinal cannot elect a pope.

Further Gracida questions the validity of the resignation of Benedict XVI. For him “there is some evidence that Benedict was forced to resign”. In this case the resignation would be “invalid and he is still the Pope”. en.news 

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Bishop René Henry Gracida, a friend of St John Paul II and Mother Angelica, expressed his 'gratitude' for the document

He told the Catholic Herald that he hoped “other bishops will sign on to this lay initiative”
Bishop Gracida referred to Blessed John Henry Newman’s history of the Arian crisis, which describes how “it was the overwhelming resistance of the laity to the Arian heresy which eventually persuaded the majority of bishops ‘who were either Arian or semi-Arian’ to support the efforts of St Athanasius and the Pope that eventually led to the condemnation of the heresy.”

Bp. Gracida:

Only God knows whether or not Francis is an Antipope.
There is no doubt that he was elected a pope, but is he a pope or is he an antipope?
There is doubt that his election was both valid and licit, there is good reason to believe that it was either illicit but valid or licit and invalid. The reason for the confusion is that the Apostolic Constitution, Universi Dominici Gregis [of John Paul II], governing papal conclaves provides for the automatic excommunication of any cardinal who participates in a conspiracy to cause or prevent the election of a cardinal. There is no doubt that Francis was party to a conspiracy to get him elected. Therefore it is a legitimate question whether or not it is possible for an excommunicated cardinal to be both licitly and validly elected pope.
All of this combined with doubt about the validity of the resignation of Pope Benedict XVI justifies doubt concerning whether Francis is THE pope.
There is some evidence that Benedict was forced to resign. If that is true, his resignation was invalid. The one person who could have ruled on the validity or invalidity of his resignation was the Prefect of the Apostolic Signatura who at that time was Cardinal Raymond Burke. The first act of Francis as Pope was to remove Cardinal Burke as Prefect of the Apostolic Signatura and to appoint Cardinal Pinto, a supporter of Francis. If Pope Benedict’s resignation was forced it was invalid and he is still the Pope of the Church but with the chair of Saint Peter occupied by an antipope.
[…]
I hope that all of this helps you to understand the complexity of the present situation in the Church.
ABYSSUM
(“There is an appointed time for everything. And there is a time for every event under heaven”, Abyssus Abyssum Invocat, Sep. 5, 2017; special formatting removed.)

Bishop Gracida and Fr Andrew Pinsent: Why we signed the filial correction