Warning: The following contains SPOILERS for The Flash season 6 finale.
The Flash season 6 finale suggested the frightening possibility that Barry Allen might have been replaced with a Mirrorverse duplicate, like his wife Iris West-Allen before him. While this wasn't explicitly laid out in the finale, it would explain Barry's out-of-character behavior, as well as events from earlier episodes.
The Flash season 6, episode 19, "Success Is Assured" found Team Flash acting as the protectors of Joseph Carver, the corrupt CEO of McCulloch Technologies and leader of the Black Hole crime syndicate. Given that Carver had threatened Joe West's life and family and had confessed to knowing about Iris' abduction and disappearance into the Mirrorverse for several weeks, nobody (particularly Barry Allen) was happy about having to protect Carver from his wife, Dr. Eva McCulloch, aka Mirror Mistress. Yet they had little choice when Carver approached the police to ask for help after Mirror Mistress easily made his bodyguards vanish without a trace. However, before they could secure Carver, they were approached by a Mirrorverse duplicate of Singh who made Barry an offer; hand over Carver and Dr. McCulloch would free Iris from the Mirrorverse.
Nash Wells teleported the team and Carver away before Barry could give the fake Singh an answer and confronted Barry privately over his allowing a known Mirroverse duplicate to get that close to Carver, much less considering bargaining with Mirror Mistress. Nash explained that all the versions of Harrison Wells sharing space in his brain were screaming that the Barry Allen they knew would never consider trading another person's life to save Iris. Barry defended his hesitation, however, by asking what kind of man wouldn't consider doing whatever was necessary to save the love of their life. While this was not addressed again in The Flash season 6 finale, Nash did raise an interesting question regarding Barry's recent behavior and it had already been suggested earlier in season 6 that Barry Allen was not the man he thought he was.
In The Flash season 6, episode 17, "Liberation," Barry confronted the Mirrorverse duplicate of Iris West-Allen, insisting she submit to a test that would prove whether or not she was human. Mirror Mistress had anticipated this, however, and replaced Barry's prismatic scanner with a copy from the Mirrorverse. When the fake Iris was scanned, she seemed to be perfectly human. When the scanner was turned upon Barry, however, it showed him to be a mirror duplicate.
The fake Iris put on a great show, insisting that the real Barry Allen wouldn't have spent a week avoiding talking to her after she kicked him out of their apartment and Barry was locked in a containment cell at STAR Labs until he finally convinced Cecile Horton to free him. While the episode implied that the scanner was rigged to give the opposite result of the truth, it was never tested on a third party. As such, there's a chance that the Barry Allen shown in "Liberation" really was a Mirrorverse duplicate.
The one problem with this theory is that there was no reason for Mirror Mistress to create a Mirrorverse copy of Barry Allen at that time. Indeed, her followers were busily setting a trap for Flash in his apartment while preparing her escape from the Mirrorverse at the same time Barry was making preparations to confront the fake Iris in "Liberation." However, The Flash season 6 finale suggested that the Mirrorverse warps the minds of people who are trapped there, and Carver claimed that Mirror Mistress was just some thing that thought it was his wife.
While Carver is hardly a reliable witness, the possibility that he is right and Mirror Mistress was insane and perhaps inhuman opens up a host of horrifying possibilities, including the idea that she might have created a fake Barry Allen without realizing it. This would amount to a major twist for the opening of The Flash season 7, with the rest of Team Flash trying to find the real Barry Allen or Iris having to save her husband from the Mirrorverse.