This fallacy sounds weird but as we will see, it is frequently employed by fantasists and mystics that have no evidences to confirm their arguments. The name of this logical fallacy is Circulus in Probando, and consists of using as evidence a fact which is authenticated by the very conclusion it supports.
A common example is this one:
"We can trust the Bible because it's inspired by God Himself."
Question: But how do we know that God inspired the Bible?
"The Bible says so."
It is a circular, invalid statement as you can see. It attempts to authenticate the very conclusion it supports.
Those who try to convince the public without showing evidences incur in the Circular fallacy when some cosmic fantasist tells us this:
"A reliable, anonymous source told him that there was a secret meeting between Extraterrestrials and Authorities"
Obviously we ask the individual how he knows that the anonymous source as reliable.
He will reply that he can guarantee the reliability of that source.
In no way the reliable source validates the statement.
It's something like this one:
"Sergeant X will tell you that I am totally honest about my personal experience with ETs."
Question: But how do I know for sure that Sergeant X is telling me the truth?
"Oh, I assure you that Sergeant X is totally honest."
Again, the circular situation denies the value of the argument. X will tell us that A is honest, and A will guarantee that X is honest.
This kind of circular fallacy is typical in the contactees' messages.
The contactee tells us that a message comes from an extraterrestrial source. ? How can we know if this is true? Simply because that is what the message says. The problem is that nothing can be authenticated by the same conclusion it supports.